In the years after WWII, several countries experimented with general-use optical sights on service rifles. The Germans had pioneered the concept with the ZF-41 long eye relief optic during the war (and the ZF-4, to some extent), and the British actually adopted the EM-2 with a permanently-mounted 1x optical sight in 1951 (only to un-adopt it almost immediately in favor of the FAL). The US also tinkered with the idea, as demonstrated by this 1x optical sight on an M1 rifle for sale at Rock Island. I was unable to find any reference information on this type of sight, but it does seem to fit the period. Perhaps one of our readers will recognize it?