I briefly considered that this might be a misprint, given that the bulb I was replacing looked exactly like the type once used in the 12V DC electrical systems of American cars. At the same time, though, I recognized that the grinder motor runs on 120V AC and that it would actually make some sense for Ryobi to not design a 120V bench grinder that requires a transformer just for its worklight.
It was at this point that I should have measured the voltage at the light socket. But I didn't. Instead, I ordered the one 120V BA15S bulb I could find on Amazon - only to discover that, when installed in the grinder, it glowed only the faintest red.
Enter the multimeter - two, in fact.
As I suspected: the voltage measured at the socket was NOT 120V AC, but instead 11.6V AC. (when set to DC, neither meter would report any appreciable voltage)
I was once again on the hunt. And miraculously, without a lot of searching, I found this bulb. Could it work? Would it work? It says AC 12V right there in the product title, so of course it will work - right? RIGHT?! Right.
Fortunately, this 3W AC12V/DC12-24V LED bulb does indeed work. It is brighter than the original and appears to be much less fragile. It flickers a bit as the grinder gets up to speed, but the flickering goes away completely after full rotation speed is reached.
All that remains to be seen is how it will hold up over time. So far, I have run it for about 4 hours.
If you have the Ryobi BGH825 bench grinder or its 6" companion (BGH625), this bulb is the one you want.