Hello all.
I am doing a repair on a Drilling that is 2 rifles over a 20 ga shotgun. The rifle barrels are chambered in a shell I can't seem to identify.
There are markings on the underside, but I am not sure if I am reading them correctly.
They look like10.6m
60.

But they may be 1C6m
60

The casting of the chamber itself gives me these measurements.

Rim diameter .540"
Rim Thickness .070"
Shell web in front of rim. .479"
Case mouth .470"
Shell length 2.365"
Bullet diameter .447" Aprox. Taken from throat in front of chamber.
Bore diameter .425"

Can anyone tell me what I have here, and if brass is available?

Note, in the writing above I tried to make the 60 come out underneath the line s reading 1C.6m and 10.6m, but the computer will not show them that way.

I hope this is understandable,