Not a drilling but a Cape Gun (one shotgun and one rifle barrel -usually side by side).

If I am reading the Proof marks correctly as 13 and 52) almost certainly chambered for a 12 bore shot cartridge and a .450 rifle cartridge.

If the rifle chamber has a very pronounced bottle neck it could well be chambered for the .577/.450 Martini-Henry service cartridge.

That was a popular combination for South African use.