The model of 1955 .45 acp was labelled a model 25 when S&W started using model numbers in 1957. So there are "pre-25's", 25's, 25-1's, and so on. This is one of my favorite S&W's ever, and a joy to shoot. If you want to know approximately when yours was made, post the letter and numbers on the butt (some people like to substitute "x" for the last two numbers for privacy purposes).
The immediate precursor of the 1955 was the 1950 Target Model, which became the model 26 when S&W went to the numbering system. It had a lighter barrel than the 1955 Target.
The army model 1917 .45 acp did not have adjustable sights, no rib, and other differences. After WWII they were called Hand Ejector Model of 1950 Military (Also called 1950 Army) with the new shorter action; this model became the model 22 - scarce, as they weren't very popular