Hi Bobba.
I know its been some time since you had the problem but I was wondering how you got on, as I have the same problem with a gun I've just bought. Its marked as 'locking piece' on the only diagram of this gun I have been able to find (http://cinedux.com/acvoke-airguns.php).
In my pistol the 'locking peice' is missing completely, and some bits of tape on the outside (where it was presumably attached at one time) are the only trace of it I still have! Did you find an original bit somewhere, have something made up, or give up?
Rex