I am not aware of any "additional frame" added to a CMC build.
Perhaps someone with more knowledge of CMC's can chime in, but, in the states cars are registered, legally identified and given licenses based on the Vehicle Identification number.
The information above is accurate as to the VIN location on a beetle frame.
CMC, or the individual builder, may have riveted or otherwise mounted a metal plate in the door jamb or other location indicating their serial number of the body or kit, but that, I believe is not used as primary identification anywhere in the US. That was done for the convenience and purposes of manufacturer and/or builder.
I don't believe that all CMC's even have the maker's plate on them, but all should have a VIN.
Subject to correction by those with more specific or specialized data.