I don't think it's a Mk1 Mac. The Mk1 didnt have any notches on the loading port.
The Mk2 had 3 notches, evenly spaced.
The Mk3 has 3 notches, 2 together and 1 further away.
From what I can see in the pics, it could be a very early Mk3. I gather they put the first ones in Mk2 stocks.
I don't know if the inner parts are interchangeable though, sorry.