Yep, mine are numbers 502 and 504. They were made in 1978 or maybe 1979. I replaced some of the caps and the Opamp chip thirty years ago. They were mostly stock until late last year when I got the bug to update. I replaced the input jacks and added tube risers last year. I had the the fuses replaced with hydraulic circuit breakers, the auto on circuit was removed and replaced with switches, and a few odds and ends replaced. So while I was at it I replace the power cords and even got a set of new tubes for them!
JD