It's a 1967 JTM 50 - known as the "Black Flag".
It's a transition amp (between the original JTM45 and what became the "Plexi"). I think Marshall were just using up existing parts as they moved from the KT66 to the mighty EL34.
So this amp is basically a JTM45 with EL34 output valves and GZ34 valve rectifier (as opposed to solid state diodes which became the norm later on).
Fortunately I have the cab - most of it anyway, I lost the back panel somewhere along the line.
Unfortunately I butchered the amp in my youth so the cab doesn't have the original gold piping - when I was 17 I wanted it to look more modern! On top of that I wanted Master Volume control and the "Van Halen" tone, so I performed my interpretation of the Jose "one wire mod" as well. Just spent a few hours undoing all of that nonsense in an attempt to return it back to original spec. I've got a way to go yet but at least I know that it works and the "precious" Power And Output transformers are functioning