Where can I find the RPO plate on a 64 Impala?

[Morgsie]

Hi, I recently bought a 64 Impala, which came with no engine. I am trying to figure out what the original engine was in the car. The guy I bought it from did not know either, as he got it the same way. I have a list of RPO codes, but I have no idea where the actual plate is. I have found the VIN number (drivers door) and the body plate (passenger side under the hood). Can some one tell where I can find this id plate?

Thanks,
Morgsie