I assume you are referring to the VIN. On my 72 it was stamped twice, both on the engine side of the firewall. Both under the heater box. One under the heater core and the other under the blower fan. The same housing covers both and in my case the housing had to be removed in order to see.
Is the "Paper" sticker on the drivers door removed/worn away? The VIN is on this sticker as well.