Hopefully, Andreas will chime in with a more definitive answer, but the number might be interpreted thusly: The D and X are the codes for the guys who built the boat (Thompson assigned each of the workers a letter). The leading "4" indicates the year of any given decade (e.g. 1944, 1954, 1964), and the "373" would be the 373rd boat built that year. Since the Thompson Sportsboat was only offered in the 1955 catalog, it would seem to indicate the boat was built in 1954, if I am interpreting the number correctly.
Attached is the 1955 catalog page showing the Sportsboat, so you can determine if you have the model identified correctly.
Cheers,
Dan