Protected 14500s- what do they fit? Constant Current regulation?

So I have a few Keeppower 14500s that I purchased for a project that did not work out, and they don't work with my lumintop tool. 14500 guys, what do these fit in? Also noticed a lot of lights have built in low voltage protection now, but many seem to be the FET+7135 type driver, which means you need IMR cells to get the most performance out of?

Also, what happened to good regulator boards? - everything seems to use a MOSFET to control "direct drive" mode for high and have a fancy micro controller with some 7135 chips to do with ramping etc - no one cares about driver efficiency anymore, outside of zebralight, muyshondt, etc?

The keeppower 14500 1000mAh fit in and work in my Frelux Synergy 2 and my EAGTAC D25As. I have both the Keeppower and the Vapecell 1000mAh and prefer the Vapecell as the Keeppowers length (longer than most other AAs and 14500s) limit their compatibility with different lights.

Maybe this will answer some ones question in the future?
I can comfirm that a protected Keep Power 14500 will not work in the new Manker E03II, it`s just a tiny bit too long so the tail cap wont go on all the way and stays locked out, it`s fine with other 14500s though.

Ended up picking another xml foursevens quark and a 14500 body - tried and true

What's the most efficient driver type for using a lithium ion with a cree xpl/xml single die LEDs anyways? FET? Buck/boost?

Constant Current, and I assume among those, Direct Drive.

I think there may be some confusion between a driver with current regulation (constant brightness) and a constant current driver. The two are not mutually exclusive, but one is not the other.