It's a variation of the Diesel series cabs, TL606(?). Loaded with EV spkrs. Search on Mesa Diesel for lots of info. As to using it as an ext cab, that depends. What's all the particulars?
EV 15Ls. If they are like the Buster 15" combo cab itself, they are rear ported cabs. Maybe it's a variation on the TL606 (would not be surprised if this is indeed the case), but it sure looks different. IMO there's probably a better 15" cab out there, but if you have that buster combo, well I get that you might want a matching set.