The Mills 75 Mk1 was introduced in 1948. The crankcase was manufactured from solid and has the angular look that you see in the first photo below. This version is extremely rare to find today.

The Mills 75 Mk2 came onto the market a year or so later and has the cast, more rounded crankcase, like the Irvine replica that you see in the second photo below.

The vast majority of the Irvine Mills 75 replicas are of the later, cast version.
Irvine did also produce a small run of Mills 75 MkI replicas – to confuse matters Irvine calls these engines the Irvine Mills 75 Mk2…..
The last of these had gold anodized heads, see photo 3, below.

The Mills serial numbers are confusing. I have seen them explained somewhere in an article, by P.G.F. Chinn I think it was. Maybe someone else out there can find the reference?

Tomas H.