The format was much simpler, usually consisting of a series of numbers and possibly some letters. It would likely include information like the model year, body style, and possibly the plant code and serial number, but it wouldn't follow any consistent, easily decipherable pattern like modern VINs. The exact placement and format might even vary slightly depending on the specific model and manufacturing changes throughout the year.
To find the identifying number, you'll need to carefully inspect the areas mentioned above. The number will probably look quite different from a modern VIN. It might be a sequence of five or six digits, for instance.