Most of those old speakers had 2 coils.The coil (called the voice coil) glued to the cardboard form in the base of the cone is probably between 4 and 8 ohms. Back then,speaker magnets sucked so to get around that, they used electromagnets,often called field coil or electrodynamic speakers.For a car speaker they might range from a couple hundred ohms to a thousand.If you go with a modern speaker,you can pretty much ignore the field coil leads and just connect the voice coil leads. If you tried that on an AC set,the kind found in a home,you would cook something. As is always the case with vintage tube driven electronics be very careful as parts of the circuit can carry pretty respectable voltage. Don't ask me how I know.To figure out what you have,You might try this site, http://www.nostalgiaair.org/. They have thousands of schematics. There are lots of sites out there that you can extract useful info from before you start tinkering including do's and don'ts.Good luck.