What you have is a genuine Oberndorf Army Type C rifle.

The rear sight maybe factory made , but could be an (American) aftermarket product.

Though you find in Speeds book "Archive" a similiar looking sight on page 527.

The barrel base for the rear sight looks as if it could also accept the normal one standing and two folding leaf sight.

If the sight should be numbered (same style as on the rest of the rifle) on the bottom it would be for sure an original Oberndorf part.

The caliber marking follows the rules of the production time.

Later ones show 9x57 on the barrel shank.