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It was not uncommon for the salesman to type or write in the code on the first inside page of the owner's manual, too. If you still have your original owners manual take a look and see if there is a code there.
Here's an example for a 1964 (car, but truck is very similar). They even provided a spot for them to write it. 3rd picture, just to the left of the date.
The 1966 owners manual did not have a given spot for it, but they would still often write it someplace on there. Of course, if you got someones owner manual then you'd have the code and could get a key made. Not exactly secure, but I think they figured that out at some point.