An addition to Raimey's cryptic response for more recent guns
I probably got this from Larry Brown?
All the Italian firearms and the foreign firearms produced in Countries that do not have a Proof House recognized in Italy, should be prooved at the Proof House of Gardone Val Trompia, before being sold. The Proof House performs a series of tests and controls of safety, to ascertain the operational qualities of the firearm and the absence of risks for the shooter. Proof of the fact that the controls of safety were performed is represented by stamps, deliberately put on the action or on the barrel. Among these, is (especially on rifled barrel guns) the date in which the arm was proofed. In the period included between 1922 and 1953, the date was indicated "in clear", with a stamp stating the entire year including the four figures.Starting from 1954, however, it began to indicate the year with a Roman number: 1954 corresponds to the X, 1955 to the XI, and so on. As the years went by, however, they realized that if Roman Numerals continued to be used they would require a very lengthy stamp: so, instead to indicate 1971 with the figure XXVII it was determined to indicate it with a mixed one of Roman numbers and Arabs: XX7. It follows XX8 for 1972 and XX9 for 1973, to end with XXX in 1974. From 1975, the Roman numbers were abolished and it was decided to indicate the year with a code of two letters. 1975 corresponds to AA, 1976 to AB, and so on. The problem is that some letters were skipped, therefore only having the entire list of the codes or proof marks you can determine with certainty the year of construction of the arm. Here is the entire list of the codes:
X = 1954 XX7 = 1971 AT = 1988
XI = 1955 XX8 = 1972 AU = 1989
XII = 1956 XX9 = 1973 AZ = 1990
XIII = 1957 XXX = 1974 BA = 1991
XIV = 1958 AA = 1975 BB = 1992
XV = 1959 AB = 1976 BC = 1993
XVI = 1960 AC = 1977 BD = 1994
XVII = 1961 AD = 1978 BF = 1995
XVIII = 1962 AE = 1979 BH = 1996
XIX = 1963 AF = 1980 BI = 1997
XX = 1964 AH = 1981 BL = 1998
XXI = 1965 AI = 1982 BM = 1999
XXII = 1966 AL = 1983 BN = 2000
XXIII = 1967 AM = 1984 BP =2001
XXIV = 1968 AN = 1985 BS = 2002
XXV = 1969 AP = 1986
XXVI = 1970 AS = 1987
