It’s all one piece which is what I wanted.
Yes, it would be a Keeppower protected cell.
I ran my Capitan on a 14500 cell, and it got pretty warm rather quickly. Because of that, decided not to do a runtime test, but surmised that at lower levels, it would definitely outperform basically every other single AA sized cell in terms of either runtime OR brightness, depending on which is prioritized.
However, based purely on paper/napkin maths, assuming that at the lower drive, Eneloops operate nominally at 1.2ishV(1900mAh), Alkalines at 1.3ish volts(2800mAh), Lithium AA at 1.4ishV(3000mAh) and LiIon at 3.7V(1500mAh), we get:
Eneloop: 2.28Wh (4.56Wh for ×2) 2.4V
AlkLeak: 3.64Wh (7.28Wh) 2.6V
Lithium: 4.20Wh (8.40Wh) 2.8V
Lith-ion: 5.55Wh. 3.7V
While the higher voltage of the 14500 may give more brightness, it's also delivering more power to the LED initially, reducing the overall runtimes. If a single cell were your fancy, I believe the Capitan would probably be the better option for you, being smaller and not requiring a dummy cell...Lithium cells are definitely the lightest options and should offer some stupidly long runtimes if the current has been dialed back and fixed at some lower value.
I have some other iteration of AA flashlight that runs at very low current, and manages to eke out about 18 hours between 15 and 40 lumens, then another few hours from 12ish to 1 lumen, and below on Alkalines. Haven't tested on fresh Lithiums yet...