It's an original Russian scope, I believe it's called an SVT scope that they could also use on the PU mount and the SVT-40 Tokarev semi auto rifle.
Some of the SVT scopes were recalibrated and have the stamp "CB" on the knobs for PU use on the 91/30 Nagant.
The PU scope has just a slightly different profile give or take. More like a straight tube.