Sorting out parts from the gunsmith estate I bought last year, ran into another one that's a bit odd to identify. Probably real apparent to a colt guy.

Have a 5 inch stainless steel barrel with threaded muzzle, has a C in square mark on the lug. I know this usually means a factory replacement part, but the threaded stainless part is ???.

Anybody with this knowledge of what it is?

Thanks!

The C in a square is a Colt mark usually on government replacement parts.

The threaded, stainless thing is new to me.