The information I have came from the Beretta website.
https://berettausa.custhelp.com/app/answers/detail/a_id/101It says they used the Roman numeral dates starting in 1922, and used a different sequence of them after the war.
Notable exceptions occurred during the reign of Italian dictator Benito Mussolini. The Italian Proof House utilized a Roman Numeral system marking representing in which year of his reign the firearm was manufactured. This began with "I" in 1922. The first Beretta firearm using this dating system was the Model 1934 pistol (introduced in 1935), thus the year was indicated as XIII (usually marked on the slide) and thus referring to the 13th year of Mussolini's reign. This practice ended in 1945 with XXIII at the close of WWII, and the chart below resets the Roman Numeral marking to "I". It should be noted that some wartime production firearms were void of any marking to indicate the production date. It is believed that this was omitted in an effort to produce as many pistols as possible for the war effort.
