If they were external brakes, they were aftermarket Rocky Mountain style brakes, not available from the factory. If you are referring to the brake shoes inside the rear drums, those are for the parking brake only and will self-destruct if used as service brakes. The larger '26-27 e-brake lined shoes are more forgiving than the smaller earlier ones, but still not designed to be used as regular service brakes.
I am not sure, but the heavy-duty TT trucks may have had external clamping brakes on the rear drums. 'Not sure why, though. Those worm drive trucks were so slow, you almost didn't need brakes. Ha, ha, ha!
M.