The A2 you have there is a later model, I'd put it between 2006 and the end of production, which I *think* was circa 2009 when the A2L was introduced.
The A2 incandescent will not run correctly on a 3.7v 16650-- the main beam will light only dimly fresh off the charger (while it's still at ~4.2v), will blacken the bulb prematurely, and fail to light entirely once the voltage begins to drop. The only rechargeable options for the A2 right now are two 3.7v RCR123s or two 3.2v LFP rechargeables. No bulb change is necessary since it is regulated and the driver will buck the voltage, but if you run RCRs there's a risk of burning out the LEDs since they are current limited by resistor, and they will be overdriven.
The A2 LED, on the other hand, is almost a completely different light with a 16650. Less heat, much much longer runtime (nearly double that of two primaries, and nearly 4x that of two LFP cells) at the expense of a bit less brightness in the main when the cell reaches about 3.8 volts, but it will keep cranking for a LONG time until you get nervous about too deep a discharge, or the LVP in your protected cell cuts you off. For some reason the secondary LEDs gain some PWM once it dips below 3.0v and the primary cuts out. I have no idea why the driver works that way, but it does.