I've just installed a Texas T distributor on my Speedster.
Its a 6V system, but when I opened the 6V box containing the coil, I notice its a 12V unit with a notice "Use with external resistor" engraved on the side.
When queried, the vendor said it is the correct unit, and said just don't install the resistor.
Is this correct? It'll operate at 6V without the resistor?
I'm good at mowing lawns.....
You're fine. Most "modern" coils have the resister built in or "internal". You can use a 6V coil on a 12V system if you use a resistor in series with the primary winding to limit the current. This keeps the winding from burning up. Apparently your vendor is using the same 6V coil for both systems with the warning to use the external series (in line external) resistor for the 12 V set up. You have a 6V set up so you don't need the resistor. I=V/R so if you double the voltage you double the current so the 12V system needs the additional resistor to get the current back down to a safe level.
Very often on 12 volt systems, they will use a 6 volt coil with a suitable resistor. Then during starting and the voltage might be lower due to the starter motor draw, they will short out the resistor so there is a good healthy spark during starting.
FWIW, typically the primary winging of a 6 volt coil has a resistance of aproximately 1.5 ohms, a 12 volt coil that does NOT use an external resistor will have a primary winding of aproximately 3 ohms.