Most of the manufacturers provided a serial number plate, typically to be attached under the hood as Keith mentioned. Of course, it was up to the builder to attach it. I know Classic Roadsters included the company name on the serial number plate. Not sure about others.
Why don’t you post a few pictures. That might help us id your car.
Former Owner of a 1981 Classic Roadsters Ltd. Duchess (VW)
2005 Intermeccanica Roadster
If you own a TDr and are not in the Registry, please go to https://tdreplica.com/forums/topic/mg-td-replica-registry/ and register (you need to copy and paste the link)