Designed by Melvin M. Johnson of Johnson Guns Inc. for their conversion of the M1 carbine and was introduced in 1963. It was based on the .30 carbine necked-down to .22 (5.7mm) and also known as the 22 Spitfire. The cartridge below matches the specs on the diagram, so I will classify it as legit. According to Dan Watters on THE GUN ZONE, there are at least 14 different variations known of the 30 Carbine necked to .22 cal.

The specimens below have basically the same measurements as the pic above