I don't recall seeing any as well. But I do know they made Mark IIA widebody combos with a 15" EVM15L speaker.
A good friend still has one. He bought it direct from Mesa around '79. It's a freakin' heavy son of a bitch!!
But the IIA widebody combos typically had a serial number preceded with a "B" and the serial numbers ranged from B301-B549.
The short chassis IIA serial numbers ranged from 3000-5574.
That said your widebody/ long chassis with a 3xxx serial number seems to be a bit unique.
