Since you didn't get any replies yet, I'll take a stab at it.
I believe that the MK2 cabinet was made from 3/4" thick stock. I am not aware of any construction sheets (plywood, particle board, MDF, or otherwise) that are available in a 13/16" thickness, but I could be wrong. There are a number of woodworking gurus around here that confirm/deny this, though. I don't *think* the original MK2 had laminated sides...
After 3/4" t-molding is pounded in the slot, it measures closer to 13/16" than 3/4", as I just checked on my cab. Perhaps that is what you saw?