Crosman model 130 aluminum breech threads

Just got this. Its got quite a bit of aluminum bits. What is the threaded portion at the front of the breech for? Was it used for other models?

IMG_20210926_114733000_HDR.1632693152.jpg

 
Thanks guys and sorry for the late reply. I deleted the account cause i couldn't change (or figure out) how to change my email. So i just created a new one under the same user name. Anyway, thanks for the clarification. I was thinking these bits were aftermarket or home made. Also aluminum, barrel band/both plugs/Rr sight, pump valve (except front half,exhaust valve and sear block).