From bristolowners.org:
Hull Identification Numbers
How to tell what the hull # of my boat is.
For example; n the transom is the #BTY245620773. Is there another number somewhere on
the boat or is the hull number in this group?
The first three initials of your "HIN", or Hull Identification Number, are the
manufacturers I.D. In this case, they stand for Bristol Yachts (BTY). The next two digits give
you the length of the boat (e.g., a 24 footer). The next three digits give you the
production number of your boat --- again, 562 means that yours was hull number 562 of
the total number produced (often this is your sail number too). The last four digits give
you the month & year of manufacture --- 0773 means your boat was built in July of 1973.