3 in Circle IP Receiver Marking ?

You had asked about the finish.

Parkerizing is a phosphate finish. There were many companies with different formulas or process.

Phosphate finishes had a wide variety of shades that range from a silvery gray to black.

We see some Bavarian returns that have been blued, others phosphate finished.

The Italian returns were a black phosphate. I do not know if all were.

With the mysterious 3 in circle marking we see different finishes. If you look at the Inland above your picture it appears to be an original black oxide finish, which is something Inland did early on before switching to a phosphate coating.

The we see some gray phosphate carbines with the 3 in circle.

It could be that whatever agency had them did the refinishing or it could have been an importer or even a subsequent owner.