I've had this old 100w JMP MKII head for 15 years, but I don't know the exact year. My tech said it had parts inside stamped from a bunch of years, oldest being 1979.
Serial on the plate front & back says S/A 03245M and the amp name on the back plate is large script like so:
Any help?
Hello Andrew,
With the serial number M it was manufactured in 1980.
Kind Regards
Marshall Support