I have at least a dozen x300's.  Some might be as old as the 170 lumen models, certainly some 500, 600 and 1000 lumen models.  Is there an easy way to tell them apart.  I would like to rebalance them putting the older ones ones on things like my training glock (so I can use the same holster) putting the brighter ones on my carry ones and my carbines etc.

If I see one in the parts  bin at the gun store it would be nice to know.

I think the lower lumen ones are just called x300s on them, the 500/600 say x300 ultra on them and I think the 1000 lumen ones say 1000 linens on the bottom of the bezel.

To add to this, the 500 and 600 lumen Ultra model had a redesigned head with the wrench flats.

600 lumen had a change and went with the current taller paddle switches.

You have to look at the serial numbers. If it starts with A, then it is the older model. Those that starts with B is the newest highest lumens model. The ones that don't start with any letter is the very old one. This is true for all X300U models, both A and B models (don't get confused with models and serial numbers).