UR18650FM 18.5mm diameter over the protection connection strip, otherwise 18.35mm.
Why would you buy ZT? If you can charge it to 4.3V, why not, otherwise it might actually be worse than FM if charged only to 4.2V.
I could probably charge to 4.3V on my Turnigy in LiPo setting, maybe, maybe to even 4.35V, I know my LiPo had around 4.3V when finished. Would have to charge it again to find out.
Otherwise you are out of luck charging over 4.2V, need a rare charger for it.
I have four pc of UR18650ZT, protected. I could not charge higher than 4.25v because the protection circuit switched off. Then the capacity was lower than for the FM version. 
Yes, I forgot about the protection!
It kicks in on my FMs as well, they do stop at 4.15V?
My TF 26650s stop at 4.21V.
So yes, you need a protection designed for 4.3V cells, and so on.
I’m glad I really stick to the older FMs, see the ZT thread and probably go there to post questions about ZTs.
Ok I foud it.
UR18650FM=UR18650F, but not always 
You have UR18650F group. And in that group you have UR18650FM, UR18650FK, UR18650FJ,…
Yes I found out yesterday, there is a thread about bigmicro selling UR18650F.
Panasonic website has only UR18650F pdf, did not notice any info there though in regard to different code names.
I think the lower the second letter the older the type of cell and potentially lower capacity. FJ 2400, FK 2500, FM 2600, my guess.
Lol I’m dumb, see I forget, I even have it in the table up there few posts up XD
I realize that these batteries can put out a lot of power, but if using a drop in that only draws 1.5A, will these batteries be OK? Specifically a Malkoff M61 drop in. Are these Malkoff drop ins regulated for input voltage or amperage?
This is weird. I posted this on 2-3-2013 and is shows listed after 2-26-2013. ?? Now it doesn’t, but I have the screenshot. Edited on 2-27-2013
Yep, these batteries will work just fine. :)
Thank you.
Thanks very much for the review! Frontpage’d and Sticky’d.