Did you notice any other symptoms of overfuelling? Smoking from exhaust, excessive consumption etc..?

The question about ECU and engine was to make sure the car wasn't underfuelling, causing detonation and overheating. Then again you'd see heat damage to pistons etc.. (had that on as S1 I owned - melted piston and bore scored).

I had a 1430cc mini once that suffered with overfuelling via a twin choke weber - that just gave accelerated bore wear and very glazed finish, but no scoring at all.

I think it is more likely to be due to foreign matter through the intake or the rings themselves damaging the bores for some reason from startup. Perhaps attach a couple of pics of the bores/pistons?