Original Item: The Vickers receiver has two top covers, one at the front which retains the feed block assembly and a longer one at the rear which carries the rear sight and provides access to the lock and other internal parts.
These two top covers are hinged to each other and to the receiver with the PIN, JOINT, COVER, MK IV and the NUT, CHECK, MK II. See Ian Skennerton, S.A.I.S No. 8, .303 Vickers Medium Machine Gun, Plate D1, items 9 and 10. These parts are very rarely available for Vickers owners.
The Mk II version of the check nut is castellated for use with a Cotter pin, and has flat surfaces for removal with a wrench.