To the best of my knowledge, the invoice from the Oliver dealer was your only proof of ownership. There were no titles, and even now when you buy a tractor, get something in writing from the previous owner showing that they received payment for the tractor. Listing the tractor serial number with your insurance company is also a good idea to prove that the tractor is yours.  Pennsylvania did license tractors for a time, but I believe they discontinued that before 1967.

Something you can get that you may be interested in is a copy of the original build card. This is the IBM punch that told the factory workers what options to put on your tractor as it was built. They also marked when it was built, and where it was shipped to, and the date it was shipped. You can order one by calling the Floyd County Museum at 641-228-1099 with your model and serial number.