Ariel Owners Motorcycle Club

I wanted to know the bolt pattern of an Ariel platform mag. The M01 seems to have four bolts laid out on a 50mm
square, I would of course be drilling them on a 1-31/32" square.
I wondered if Ariels third bolt was in the centre of the square?

Yes.
The 3 holes are on 31/32" centres in both directions, that means the outer 4 are on a 1-15/16" square.