That's a Mark IIB. The serial # is actually 9XXX, my Mark IIA is #4065 and was made in August of '79.
The initials under the power cord appear to be "MB" for Mike Bendinelli, the head tech at Boogie. That may indicate that it was upgraded to the Mark IIC spec's, but it's impossible to tell from the outside.
It's somewhat odd that it has a 15" speaker. I can't recall seeing that in another one of those amps.