Otto:

The "R" is applied to rifled long arms. The star over "S" is the inspector's mark. The crown over ELG in an oval is the definitive proof mark for a rifle having undergone single, final proof. The "11.1" is not the pressure, it's bore diameter in millimeters.

How did you decide that this gun is a .500/.450? Are there caliber markings on top of the barrel?

I presume you mean that .451" is the groove diameter? That sure is tight for a .450.

This is a good example of why it is often impossible to say for sure with a Belgian rifle in a caliber that was offered as both black and nitro. A .450 at 6 1/2 lbs, the answer is pretty clear. At 8.5 lbs, marked as proved for jacketed bullets, it could be full nitro. They made some that light. J. J. Perodeau had a no name Belgian Guild DR, a .500 Nitro, in his shop a while back. It weighed 9 lbs.