That pin is the base of the rear sight as Peter has said, it also trues up all the parts as the photos I have show the head casing, trigger plates and tube as separate items being jigged and welded at the same time rather than a pre assembled head casing and side plates being welded to the tube. On the Mk2 gun, there is a hole in the tube for this pin rather than a slot as on your Mk5 and as you can imagine, this proves you couldn't fit a pre assembled head casing onto the tube as the pin wouldn't go into the hole.

Also the V changed to a 5 in Oct 1944 I think it was.