Those are Belgian proof marks, put on literally millions of firearms over a span of decades.
It would be helpful if you were to show a picture of the entire gun.
OK, a little more info...
According to this PDF file (ahttps://store.bluebookinc.com/Info/PDF/Firearm/Proofmarks.pdf) that's a Belgian proofmark, used between 1811-1892, for blackpowder rifles designed for importation into Britain. It is listed under the British proof marks section.
The corresponding Belgian proof has a crown sitting on the oval.