It's a long shot but here it goes anyway....I'm repairing my uncles childhood air rifle. A late 1960's Gamo ASI sniper repeater .177 air rifle.
This break barrel air rifle is a normally has a magazine tube to auto-feed pellets. That is the missing part I am looking for. I believe the same part from a Paratrooper ASI repeater will also fit.

Well, if anyone here has one or knows of someone with one, please message me, cash waiting!