Well, I don't know the total "code" for Catalinas, (O'DAYs and CALS I do mostly know!) but to use my O'DAY as an example, "M79E" is how my boats HIN ends and the explaination should be same for Catalina.
M79 = 1979 Model Year, E = month code for when built in that model year. Model years ran August to July (for 1979 that would be Aug 1978 to July 1979) so.... if your HIN ends with M79D that would be 1979 Model, built in November 1978 (A = August, B = September, C = October, D = November, E = December,......... K = June, L = July)
daleyellin: I think Rudy might very well have either solved your question or at least given a darn good possible answer. That would make sense based on how other brands of sailboats with more than one factory numbered boats. The West Coast plant may have been unsure what the Class# should be and just went with a "fudge factor"! There certainly wouldn't have been another 222 with Class# 0000, so they were sure they wouldn't accidently repeat a number.
However, one would think they could have phoned/faxed the Fall River, MA plant to verify what the proper Class# should have been to insert into the HIN? Oh Well!