What is the difference between the 501 and 502 series of lights?

These are probably what you are looking for:

black - http://www.manafont.com/product_info.php/ultrafire-wf50x-flashlight-tube-metal-bezel-black-no-led-emitter-p-6064

...and silver - http://www.manafont.com/product_info.php/ultrafire-wf50x-flashlight-tube-metal-bezel-silver-no-led-emitter-p-6065

I've had both of these; the switches didn't handle a Direct-Drive XM-L dropin, but the bodies and finish were good quality. $8.12 each...

Yea those are the ones ! I put the old WW drop-in Manafont used to sell in them with a bunch a Al foil. They have worked well for me. I did upgrade the 'o' rings in them.

If the light changes modes when shaking, then the driver spring is likely one of the short nangj springs. I always replace those with longer springs to avoid inadvertent mode changes.

A solarforce L2N with steel belt clip as I metioned to a friend who helped me decide on the solarforce host , was I am putting a small block chevy engine in a Ferrari chassis ; referring to putting a lightmalls p6oxml2-u2 in a solarforce host .Well I would like to thank everyone for their input and helping me make this decision, and I would like to wish everyone a great weekend !

Agreed about the longer springs, but if you don’t have time/parts, a small disk magnet or two on the (-) end will help. (Fasttech sku 1090501, for instance.)

I liked my 502b just fine but I have had 3 different 501b’s and had trouble with the tailcap retainer ring working itself loose on all three.

They'll do that if the ring contacts the rear of the battery tube before the tailcap threads bottom out, when you unscrew the tailcap the ring sticks against the tube and unscrews too. See if you can fiddle with the switch stack-up parts to get the ring to seat a little farther down in the tailcap.