According to the USCG:
"Prior to Nov. 1,1972 there were no requirements for a “HIN” to be assigned to recreational boats. Manufacturers used mostly serial numbers. The serial number assigned to the vessel by the manufacturer was unique to that manufacturer, but had no prescribed format beyond that specific manufacturer."
So prior to 11\1\72, individual manufacturers used their own system of numbering and the current system of numbering didn't come into effect until 8\1\84. Between '72 - '84 they were developing the system we have today.