Aah, the amp with the ‘urinal cake’ epoxy inside. I almost nabbed one off eBay for 850 a few yrs back.
Pretty rare amp; I’d imagine 3500 is fair with prices where they are nowadays.
If it is 1981 or earlier there will be a letter at the end of the number. My 81 is Letter N which is 1981. After 1981 the Letter was moved before the number.