Short answer is it depends on the state that issued them. I believe many states used metal year tags, at least in the 1940's and 1950's due to metal shortages. For WI, there is a decent webpage on
Wisconsin License Plates that has a nice picture of 1940's and 1950's (and at least up to 1960 from what I could see) plates showing the year tags. The tags weren't bolted to the original plates. They had tabs that fit through slots on the main plate and got bent over to keep secure on the original plate. These are easy to see in the picture on this webpage. Guessing you may be hunting for Year Of Manufacture (YOM) plates for your truck???
