I also presumed it was held in by screws, but the ones visible are just to hold the outer plate to the lever housing. I have a new unfitted one and can't for the life of me work out how it is meant to be held in. The collar visible on the other side of the stock is just a pivot bearing for the lever shaft. I've looked in my Greener book but there are no diagrams or such, just a brief mention.
I tried looking for the patent but with no luck.

Afro