It is not unusual for the HIN to get sanded off the transom if the boat
has been repainted. However, as suggested by others, the HIN is shown on the USCG Documentation
in the 3rd block top row labeled "IMO or Other Number."
If that block is blank, then there may be something "fishy" about the boat and who owned it and who sold it as the HIN should appear on all relevant papers (like the Bill of Sale) during the sale
process to properly identify the boat during the sale
The USCG Documentation
number is normally permanently cut/ground/carved into the hull somewhere in the boat as an identifier as to the boat being USCG Documented. You need to find that number to see if it agrees with what is on the USCG Documentation paperwork you were given.