Is the serial number plate missing? It's usually in the back of the amp just above the power cord. Should be in the format of R-XXXXXX.
In one of my earlier posts in this thread I described the revisions, you can usually figure out the amp revision just by looking at it, and that will give you a clue as to when it was made.
Also, my amp has the manufacture date written inside of it (August 1993). If you pull the chassis out and flip it over (careful!) you can probably find a similar date inside.