On the inside of the the extension piece that slides into the receiver, towards the inner most section, there is a dot of metal.
What is it for ?

Good question. I'm going to go dig my wife's out of the safe and study it. I spit-wadding here, but might it play a role in shell ejection? Tripping them out the ejection port?

That's the ejector, bro..

Ah, I didn't use shells while trying to figure it out.

Thanks.
I was trying to figure out if a 3.5" barrel could be cut to a 3 " extension length, and what that bump was.
Ain't gonna work, unless I can weld another and shape it.

And such a cheap one at that...but it works.

Doc