Bob, your gun is likely one of these:
MODEL 409 PB - 12, 16, 20, or 28 ga., 27, 28, and 30 in. barrels, various chokes, double triggers, plain extractors, checkered pistol grip stock. Mfg. 1934-1964. MODEL 410 E - higher quality, auto ejector version of 409 PB. MODEL 411 E - similar to 409 PB, with false sideplates and finer finishing. Mfg. 1934-64.
All these guns are great shooters...while I've never seen unit production statistics, the early 50's seem to be very prolific based on anecdotal observations.
This site has info on italian proof marks & may be of interest to you:
http://www.bancoprova.it/index.php/en/proof-marks/italy.html