I have three ACR 4F strobe lights, an old (80's or so strobe light) that have been modified to run on two CR123A batteries instead of the original equipment mercury battery. I note that my recent flashlight acquisitions running a 18650 battery can also accept two CR123s. So, is it possible to stuff a 18650 into the 4F and operate it properly? Or will the whole thing melt down?

I suppose I could just try it, but, as always, I am extremely prudent and seek expert advice....

CR123A batteries run at 3V and perhaps 1400mAh. Two of them will double those figures. An 18650 runs at 3.7V and as much as 3400mAh.

I don't know enough about your modified strobe to estimate how it would operate at a little over half the voltage. I'm guessing that if it's not designed for different voltages, you're best off sticking with CR123A primaries.

As CM mentioned, "it depends". I have several older CR123 lights that will not accept an 18650. The 18650 is actually slightly larger in diameter than the 123s. If your strobes haven't been designed to be co-compatible, then I doubt the 18650s will even fit in the strobes.

Be warned that there are many narrower 18650's that will fit in a light designed for 2x CR123A cells (inline, not side-by-side, of course) but still won't work. While I can't immediately think up a way that an 18650 that fits properly would damage a light designed for 2x CR123As, that doesn't mean that it won't. If your light isn't designed for a specific cell, don't use it.