Sweet looking 19’6! Nice find!!I believe federal law did not mandate HIN numbers for boats until 1973.You should be grandfathered in to register without one. I also have a 1972 19’6 and our Secretary of State(DMV) in Michigan accepted what I had on the plate. E300 for me. Luckily for me the the associate for the Sec of State was familiar with the law and gave me no trouble. Maybe you could try referencing that particular law when registering? Good luck