A 1966 probably doesnt have an original factory HIN, but might have a serial#.....
what state are you in? States have different rules/regs for boat documentation, some easy, some not and some that are extremely difficult to deal w @ 50yr old boats
might look under the splashwell and/or dash if equipped, some makers put 2ndary serial #s in those locations.
HINs werent mandatory until early 70s....
some states will issue a new state issued HIN at title/reg transfer due to sale of a pre-72 boat that never had or needed a HIN