The manufacturers often scribed the HULL NUMBER (a "serial number" sequentially assigned to a hull by the manufacturer) in out of the way locations.
My Cape Dory has the hull number and date of manufacture inside the rear lazarette starboard side where a fuel tank would sit. My Capri has the number under the ID tag on the inboard of the transom. Check several areas and under plates.
Starting in 1972, manufacturers were required to have a Hull Identification Number (HIN - similar to an automotive VIN). The first three characters define the manufacturer. The next five characters describe typically the model and the hull number. The last four characters describe the month and year of manufacture (two principle formats for date).
One HIN copy was to be on the outboard STARBOARD TRANSOM. A second HIN copy was to be placed in an out of the way location and is often hidden.
If a HIN is not found, States will assign a HIN. The states' HIN starts with the two letter state code.