The hull number should be on the starboard side of the transom hust below the rub rail. The number is engraved into the hull, just visible. While the number may not be crisp, there should not be any indication of modification. The following is from the Catalina owners association:"According to my paper work the hull identification number is "CTYN0773M77L". What does that mean?" The hull numbering, as required by the USCG, contains 12 characters; 3 groups of four. The first four are manufacturer and size, i.e. CTY for Catalina Yachts and "N" for 30, "L' for 27, etc. The next four are the serial number of the boat (only the C22 exceeded the four spaces allowed with 16000 boats built-we dropped the "1" at the front). The last four are the year of manufacture and/or model year. This coding changed over the years.If your boat has the old number system. The first "M" means modified. The next two digits are the year built. The final "L" is the month "July" in the modified system. The later system started with an "A" which is Jan, "B" is Feb, etc. The second number is the year built (4=1994) and the last two numbers are the model year (95=1995)