They've been around for a long time - still made, check out Midway for some current versions. I have a few on older rifles - Remington 141, Remington 81. Pretty good set up. Large screw into the base, turn the wheels to loosen. One of the rings (rear IIRC) has an off center screw set up so you have some lateral adjustment.
No surprise that it's on a sporterized 1903, very popular about 50 years back.
You need a steel Weaver or old Herters scope on that for the period correct look. :wink: