Most of the changes between the 4-speed TS250 engine (MM250/3) and the 5-speed TS250/1 engine (MM250/4) were mechanical. Larger main bearings lubricated by petroil not gearbox oil, 5 speed box and revised gearchange mechanism plus new crankcases to hold these bits. The combustion chamber was still the same design in use since the ETS250, but the plug was moved to a central position.
The exhaust was unchanged, so was the cylinder barrel and therefore the port timing. Carburettor changes were minimal: main jet down from 140 to 135, and no cross-hole in the needle jet.
The TS250/1 annex to the workshop manual lists the changes, but says nothing about the change to the inlet pipe. It is mentioned in the parts list, but understandably this doesn’t explain why.
I was hoping that MZ would have said something about it in a press release or dealer bulletin, and that someone would remember seeing it. I guess you win some, you lose some.