i recently bought 2 of these reflectors, one for my 13.99 501b and one for a friends 501b.
they are machined out on the backside to clear battery wires better and to start the base of the reflector level with the base of the LED. i like them. they make a xm-l throw a little bit better and they make the flashlight look cleaner down by the LED.
they also dont have a step at the bottom of the reflector.
I like these too... They are a good replacement for the reflector on that 3-mode drop-in that accidentally gets a thumb stuck in it causing permanent damage to the factory reflector. Mine even seems to be better than the factory reflector in fact...