Those were mini stack heads that came out around 84-86. They were the budget stack of the JCM 800 line, since they were 3/4 size and solid state.
If you noticed, the Jubilee also included the mini stack line, only they had the same tube circuit as the bigger fullstacks. MOSFET Mini stacks didn't last long....they kind of evolved into the Valvestate line.
Stacks = fullsize
Mini stacks = 3/4 size
Micro stacks = the little tiny stacks - Lead 12 SS.