I've written below what I wish I would've encountered when I was starting out in white LED flashlights. I learned everything below in the wrong or reverse order, so hopefully this will be helpful to you and/or others investigating the same topic.
The Solarforce L2* line is based on the (American) Surefire 6P. Surefire was popular before white LED's were a big thing, and incandescent was king. When the white LED rage hit, Surefire didn't seem to notice. Although before my time, I surmise that Solarforce made most of its fortune providing accessories for Surefire which Surefire just wouldn't make itself--especially LED-related accessories. The big difference for flashlight bodies in LED versus incandescent is voltage. Powerful incans seem to like a higher voltage (I'm not into incans myself), like 6 volts on up, whereas LED's work very well on single Lithium-type cells. For flashlights, this means incandescents bodies are usually going to use 2 or more cells, and for LED's, 1 cell is going to be more efficient. I believe the "6" in "6P" means 6 volts (6P is the 2-cell version), because I see the pattern that the "9P" is their 3-cell model. And when I say "cell", I mean lithium primary, or 3.0 volts; Surefire still doesn't seem to know what a rechargeable is, either.
So back to single cells. Well, the Surefire bodies are made for CR123 cells, and typically 2 or more of them. 18650 cells (commonly used in laptops) were becoming available to mere mortals, and they had more capacity, and were about the length of two CR123 cells, but were just a bit too wide to fit into the Surefires. So while some people were boring out their Surefires with a drill press to fit 18650's (still going on today), Solarforce was making inexpensive battery tubes to swap into Surefires, and other accessories like cheap clicky tailcaps (the 6P and other models come with a twisty/momentary switch, which I like by the way).
Many people spent a lot of money to get a Surefire, and loved the quality, but really wanted the benefits LED brought. So while Surefire seemed to expect everyone to stick with (their) incans, the world was moving on, and using their drop-in design to sell LED "bulbs" to upgrade Surefires.
I'm not sure if Solarforce started out producing LED upgrades, or tubes, or whole flashlights. But at some point, but 6P 'copies' appeared on the market, but I use the term 'copy' loosely, because they're not identical, and for LED users, the Solarforces are actually in general superior. It was and still is called the L2. Solarforce was and is not the only one either, but probably the best overall. Some things most LED users will consider superior over the Surefire 6P: the L2 has grooves in the head for heat dissipation, accepts 18650 cells OR two CR123/16340-sized cells, comes with a [usually reverse] clicky switch (although as stated, I personally prefer twisty/momentary, if not forward clicky), comes with an aggressive crenelated bezel (which I also see as a downside, but many like that), and they make extension or even 'Mini' (single 16340) battery tubes. And of course, the price, being made in China. Downsides are: quality and ruggedness are not as high (fit and finish)... and... I'm running out of downsides!
Surefire still somehow seems to focus on incandescent, by the way. They remind me of Mag Industries, who have great design, machining and quality, US-built (you're outside the US but that can mean a lot for people here--they do put an American flag on the front of their packaging too), but just don't seem to notice technology passing them, hence the many copies and mods.
You have a lot of flashlight manufacturers whose names are takeoffs of "Surefire". Trustfire, Ultrafire, Sacredfire, Pants-On-Fire (just kidding). You can have some fun coming up with spoof names. So, for people who try to minimize the influence of this company's designs on today's products, they're living in a fantasy world. It was also Surefire who invented what we commonly refer to as the "P60 drop-in", the drop-in assembly being part "P60", as far as I know. Other Surefire models also take such drop-ins.
Ultrafire got a lot of play with their 50* models. I assume those models were sequentially released, and you can see an evolution in quality, and following the 6P design more and more closely with subsequent models. The 501b only bore Surefire similarity in that it could use P60-style drop-ins. Other parts (head, tube, and tailcap) are not compatible. Same deal for 502b, but seemingly a bump in quality of design. 503b seemed to be an effort at copying the 6P design, at least cosmetically, but parts are reported to not interchange well or at all. The 504b (at least the "old" or original version? or maybe the 'new', completely different and crappy design recently discussed is a fake? dunno) was a serious attempt at Surefire compatibility, but still not acheiving the compatibility of the good old Solarforce L2, however it's a bit less expensive and has a few advantages, like a smooth bezel ring. Solarforce machining is pretty tight too (a good thing but can get fatiguing), so the Ultrafire is actually easier to unscrew the head on, if that matters, with its sloppier threads, ha.
I and others have confirmed that you can interchange Solarforce L2 and Ultrafire WF-504b heads and tailcaps. My 504b smooth bezel ring fits on my L2 head, but the supposedly Solarforce-brand smooth bezel ring I got from Lighthound does not screw into my 504b, although both bezel rings look identical. There are also subtle differences between the L2 and L2p--it's not just finish.
The L2P proudly states right on the body, "HAIII", under the brand and model name (just looked at mine). I'm personally not overly-impressed with HA-III, it has a matte (non-shiny) look and feel, which most people do like. Supposedly it's more durable, but I kind of like the look of a well-used tool (that sounds kind of wrong). Supposedly there is no "HA-II", just Anodizing type II, and type III is considered "hard anodizing". I've heard there is no real definition of HAIII, but not being an HAIII fanatic (even after owning one), I don't really know for sure, or care much.
Anyone with the massive benefit of BLF, who hasn't purchased yet and is reading this, might as well skip straight to Solarforce L2 or L2P, and select a drop-in they think they're really going to like, instead of playing the "I'll get the body for $5 more if I get this combo one with the mystery drop-in" game. You will pay more money, but will be more likely to end up with something you're equally more satisfied with. And actually, you'll probably save money, because I guarantee you, if you buy ones of those cheapy combos, you'll probably still end up doing what I mentioned above anyway at some point. To me, a big deal comes down to do you want or not want blinky modes. Many very good options are available now. Also remember there are flaws inherent to the P60 style drop-in design, but the "Lego"-like customizability tends to be so compelling as to outweigh these objections.
Flashlights designed to be high-powered LED torches from the ground up are better in design, but also commonly larger, like the C8 and its many take-offs. In the end, what you spend on a good host and a drop-in you really want, will likely be equal or more in cost than buying a quality purpose-built budget flashlight, but the upgrade path is going to be much better on the Surefire-style lights.
I had absolutely no idea of any of the above when I started out. I resisted the 6P-style's popularity, because I just thought it was ugly, and I didn't know the upsides. I learned the above Surefire story in reverse order, hunting in vain for the only budget flashlight ever really commonly broached on candlepowerforums, which was this elusive L2, which most of the flashlight snobs nevetheless seemed to own. It took me awhile to even figure out what this "DX" was that people were talking about (DealExtreme), and by then Solarforce had broken their relationship with DX (or, perhaps more accurately, made sure that DX would not sell its products nor Solarforce counterfeits--I don't know all the details--and yes, I know the term 'Solarforce counterfeit' carries a little irony, but still there is a dif). I learned the hard way that cool accessories like flashlight mounts, pressure pads, and alternative tailcaps followed heavily popular models like the Surefire take-offs. Other 'strong' model varieties are C8 style, and WF-500 style (both larger).
Despite my hours on CPF, there was nothing like the resource here for less-expensive and non-custom models. We here have now collectively realized that CPF was run as a for-profit organization, and therefore the advertisers and authorized sellers were favored, both officially and by sympathetic culture, and I'll skip further comment on its moderation and overall culture, as retort is abundant here. And, 'budget' items were a lot less budget back then, and choice was smaller across the board--including batteries. If you're here, it's a great time to be getting into LED flashlights. All of the information above is scattered about on this board, and your question has been asked numerous times, but that is the bigger picture or background, to my knowledge. Hopefully, someone will refer back to this when the question comes up again, or someone is searching out there on the internet... although you have 261 posts and joined here around the time I did, not having talked directly with you yet, I say happily to you, and stoppers-by from the information superhighway: hello, and welcome to BLF!