I must be looking at a different rifle than you are! The rifle I'm looking at, in your post, is not a boxlock, but a back action hammer rifle! I don't see a third fastener, needed for a Greener cross bolt, either.
Was the so-called "Sellous grip" installed at the factory, when the rifle was built, or is it a repair, done at the factory later, the latter is my guess? How does it shoot?

http://www.hallowellco.com/abbrevia.htm

Check this out, and scroll down to "BOX LOCK", then to Greener cross bolt!