I have this which might be what you're after. Can't locate M number but 31.8 STX of some sort!
Scruffy but would clean-up, very strong spring. £8 posted if any good.
I’ve got this one in the parts box. Think it actually came off a P7. Needs a polish but the STX stuff comes up lovely. Sticker is peeling but it’s got to be 20 odd years old
Think they might both be top pull - your pic is a bit blurry - just MC32 v's MC34 maybe? If so I think the MC34 is later with a different design, but I'm sure there's someone around with better knowledge.