I did some investigating in the Coast Guard database of registered vessels, looking at the range of the Catalina 320 hull numbers versus build year and model year. I also looked at the self-reported HINs from the Catalina 320 International hull numbers. The hull numbers below are the range I found for each year, based on the HINs that were listed. The numbers between the last shown for a year and the first for the following year might be in either of them, as there was no data on those.
Because the HINs are self-reported to both the C320 web site and the Coast Guard, this data may be not entirely accurate. People frequently mis-read their HIN. Catalina may have done some messing with numbers at the ends of build and model years, as well, so specific hull numbers may be forward or back a year. For example, hull numbers 101 and 102 were built in January of 1994, but hulls 103-105 were built in December of 1993. Hulls 101 and 102 probably had some issue that caused their completion to slip into the new year while the 103-105 hulls were completed. There are certainly some HINs in the Coast Guard database that can't be right.
Just to give you an example of how to use the table, if you know a boat is hull number 600, it would be built in 1998, but was a 1999 model year boat. It also means that you can construct all but one character of the HIN...CTYC0600?899, where the only question is what month the boat was built in. The question mark would be a letter between A and L If you search for CTYC0600 on the Coast Guard web site, you find that the letter is K, which means it was built in November.
This info can be useful if you are purchasing a Catalina 320. If they tell you
the hull number is 600, but that it was built in 2003, you know they are
mis-informed or lying. Boats years in advertisements should list the model
year, not the build year (just like with cars), so hull 600 should be listed
as a 1999 Catalina 320. Catalina switched to new model years starting on June
1st of the previous year.