Strange Question on HIN
 

I'm looking at buying a used boat. The HIN is confusing me. I was
always under the impression that the first 3 characters of a HIN is the
manufacturer identification code. Mine, however, are only numbers.

Here is the HIN: 100520677-m21

The boat is labeled as, and appears to be, a 1977 Mako 21. Any help
deciphering this code would be greatly appreciated.

Thanks in advance.