Need help finding 1892 bolt sleeve part

[Robidouxs]

I have two 1892 bolt sleeves which have threading below the flip safety in the rear. The threading is still intact and is usable. What is the name of the part that would go there and where could I find it?

[Dick Hosmer]

Unless I am completely misunderstanding you, that is simply a machining feature found on the earliest sleeves, and, they are just rings, not threads, and, nothing else "goes there".