In 1967 the JMP (Jim Marshall Products) panels were introduced. In July 1969 Marshall dropped the Plexi face plates and switched to the gold colored brushed aluminium metal face plates as we know them still today.
In 1975 the Master Volumes were introduced.
The US distributor at that time (Unicord Inc.) decided to switch to 6550 output tubes in favor of the EL34's. Too many Marshalls were coming back under warranty with defective output tubes. Apparently the EL34's didn't like to be knocked about while being shipped from Europe to the US of A. Instead of fitting new EL34's Unicord fitted 6550's, thinking that these "heavier" tubes would last beyond the warranty period. This in itself was true, but many American guitar players were wondering why their Marshall amps sounded so different from their European brothers. Some people actually like the sound 6550's in their amps but most guitar players prefer the original sound of EL34's.
This amp was in production from 1975 up to 1981. Strangely enough, mine is from 1982 according to the serial number.
Specifications
s/n: 00904P