Just Mk.2 as Z is the TOPS style coding, used as the last part in the code. A Mk.2 TSO is AC2Z.
Probably the best place to check for 'errant' lettering (Inter-City, Scotrail, ScotRail) on these coaches is to browse through contemporary 1980s magazines: RAIL Enthusiast, Motive Power Monthly, Railway Magazine, etc. or perhaps the Platform 5 coaching stock books.