Aloha BNW,
Here is some information from the TRA Judging Guide 2004:
"Battery Cables (Battery Leads or Engine Leads). Three variations are listed in the parts manual corresponding to three changes to the wiring harness; no further details. Original cables are characterized by lead caps that attach to
the battery posts. Through the center of the lead caps, a sheet metal screw is turned into the top of the posts. The lead cap connectors provide marginal contact with the posts and some dealers reportedly replaced these with conventional clamp connectors.
* Earth Lead (ground)-- This is a round, woven cable. No sheathing.
* Positive (Solenoid) Lead -- This cable is sheathed; a rubber boot appears on the solenoid connection
Starter Motor, Solenoid. The same solenoid is fitted to all models. The solenoid fitted to TR4s is interchangeable functionally. The TR2/3/3A/3B solenoid should be fitted; it is cylindrical with rubber button for underhood startup. The starter change at TS50001 is the only change. The starter motor casing is painted matte black. Solenoid is
unpainted."
My guess is that the cables from battery to solenoid and solenoid to starter are sheathed in black. Looking at some under hood photos of Don Elliott's very original TR3As, confirms they are black.
I would suspect that the relatively short cable are about the equivalent of about 4 AWG, but a larger diameter cable might be a better choice electrically.