VIN Standardization came in 1981, as others mentioned. But there were VINs prior to 1981, and when a manufacturer didn't use a VIN, they used a serial number. Each manufacturer had their own process for establishing some sort of VIN and/or serial number conformity, within their product line, as noted in the Airstream example above.
As someone involved in Public Safety/Law Enforcement prior to retiring, I ran a LOT of VINS/SN in my career. Finding them was sometimes difficult, and finding something in the various state databases was even more of a challenge. Also, in the early days, NADA and NHTSB published VIN configuration books (pre-internet), that showed the VIN location and the VIN structure for most manufacturers. You might want to check your local library to see if perhaps they have some of those old books, probably in archive. Because of the plethora of formats, the VIN Standardization Act came into being.