With thanks to Mike D for these superb pics. Mike says about this rifle:
The latest addition to my little airgun fold is this Match version of the Diana model 50, in this case in .22 caliber as imported by A F Stoeger of New York, under their "Peerless" name. (Stoeger apparently just didn't believe in that wimpy little Euro .177 bore, all their larger Dianas were offered in .22 only).
The 50M was the most expensive Diana air rifle throughout the 1950's, designed to compete in early 10-meter matches against guns like the Walther LG 55 series and Weihrauch HW 55. It was discontinued in the early 60's after Diana simplified their sporter line and began introducing recoilless target rifles.
I'm only making an educated guess, but the features of this gun date it to around 1956 to 58. It is in astonishly good condition, and in perfect firing order.