Unusual Sported Ross

A friend brought me a rifle to get my opinion about it.
It is a Mk. II Ross creatively end extensively sporterized by a skilled amateur. Decent workmanship.
It also had a telescopic sight mounted. A base was handmade which fits over the barrel breech, with a dovetail on top. The handmade scope mount slides onto the dovetail. The mount incorporates three rings to support the scope.
It turns out that the scope is a Soviet PEM dated 1939. I assume that the rifle was set up in the '50s.
My friend has asked me about valuation. I suggested the rifle alone might bring $300, or so. Sported Rosses don't attract all that much interest.
The addition of the scope puzzles me. I assume that it has value in and of itself. The rifle with the scope is very interesting.
Any comments or suggestions? I don't have the rifle at hand, so no photos are available at the moment.

If the PEM is in original finish with the original markings it is worth at least twice what you quoted for the rifle. If this is one of those German fittings where they polished off the Soviet markings and soldered on some sort of claw mounts the value drops by at least half.

Sounds like an interesting combo, photos would help.

The scope appears to be intact, blued finish, and as far as I can see, completely original. It has a star, hammer & sickle, etc., No. 5, with a serial number and 1939. Heavy thing.
I should have taken some photos.
In a way, it would be a shame to break up the set, and sell the scope separately; it is an interesting setup.