Take off the lid and the game is up.
Just above the keyboard ribbon cables is the 80186 co-processor board which, when switched on, turns the machine from normal 6502-based Master 128 mode into 80186 Master 512 mode.
This machine itself is a good case in point of a misdiagnosed Master, as it was given to me some years ago during a monitor collection run. "There's a broken Master 128 over there", they said, "but you might as well take it with you."
Imagine my delight on getting it home and taking the lid off to find it was a Master 512 all along! The only thing wrong with it was that the ROM expansion board on the right-hand side of the main PCB had come a bit loose and was preventing our friend from booting up.
Don't be confused by the ROM expansion board, by the way, as it has nothing to do with the Master 512 at all. It just happens to be another add-on inside this machine.