M = Mystery. Seriously, you might be right that your '73 model year hull was built and numbered in a (pre-1972) non-standard format. And then the factory didn't change it after the standards took effect. Yours does not match either of the below formats. I'm sure you've done plenty of searches looking for the answer.
1972 to 1984 Manufacturer HIN Formats—HINs were formatted as follows:
| XXX | 12345 | 1280 | X |
|---|
| 3 letters identify the manufacturer’s identification code | 5 letters or numbers (serial #) | Month and year of manufacture or date of certification | Additional optional characters for builder’s purpose |
| XXX | 12345 | M | 73E | X |
|---|
| 3 letters identify the manufacturers’ identification code | 5 letters or numbers (serial #) | Letter code indicates information to follow | Month and year of certification | Additional optional characters for builder’s purpose |
copied from the California DMV
