rrausch is correct.
But you can use the 24v filter on a 12v no problem, but not the other way around. I use the 24v Stratapore filter, LF3894, on my 12.
Stratapore is supposedly Fleetguards best filtration media, but I would guess the filtering abilities to be very close to that of the Microglass. They do not make a Stratapore filter specifically designed for the 12v, and as I said before the 24v filter will work with no ill effects (and this was confirmed by Fleetguard).
It seems to me they would just eliminate the 12v filters and use what are know as the 24v filters now, for both the 12v and 24. It sure would simplify inventory and the MFG process.