No, it has nothing to do with cross bolt. That screw is present on another Grant I own which lacks the cross bolt mechanism. None of these old Grants are stocked to the fences and some have that screw. I'm guessing, but I think that screw may allow easy access to the strikers. I've wondered the same but I'm only guessing.
I think your guess is about right. I have a similar William Powell and (from memory) that screw holds the strikers captive. Disc set strikers are not fitted in the standing breach - and the strikers are removed by first removing the lock plate, then removing that screw and withdrawing the striker backwards into the lock aperture.
