Nice little rifle. Great Condition

Given that there doesn't seem to be any info on the cartridge/caliber, could the rifle have been a .300 Sherwood originally with the bbl marking .300 S

Then some one lengthened/enlarged the chamber to what you have and added the 'L' to the caliber marking in between the .300 and the 'S'. So you have .300 L.S.

(The .300 Sherwood was also known as the .300 Extra Long.
So making a .300 Long Sherwood wouldn't seem right unless you didn't know better. Or just had to do something with the caliber mark.

Just a thought. Bullet dia at .300 seems right.
Doesn't make any different, it surely shoots well!

It's great that Rocky Mtn can make up brass cases for instances like this. Brings an orphaned rifle back to life.
Thanks for the explanation of the filler placed below the powder in the case. Never have heard of that before, never thought of it before actually!

Nice shooting!!