My guess is the previous owner signed over the car and the UID did not go to the DMV and register to get new tags. I know at one time that the first 2 letters on an Oklahoma tag identified the county the car was registered in. I don't know if or when that changed. But if it was still the case when the car was registered it's probably Atoka County (AT). I should add that at that time Z was Tulsa, X and Y were Oklahoma County. I'm going back to the 70's. May be worth looking at, just a hunch on my part.