We do know a few things based on survivor cars and vintage magazine road tests.
Rear axles were painted as an assembly; cover and brake drums in place but no lines. Pinion flange was masked. Paint was a glossy black. Yes the drums were black too.
Leaf springs were not painted. The forging process left them with a mottled black-grey finish.
The sub frame was first dipped in a cleaning solution, then dipped in an asphalt-based coating that dried to a semi-gloss black appearance. I work with a guy that was a line supervisor at the AO Smith frame plant here in town. We had a good laugh over the filler-prime/wet sand/paint process some like to think of as "original". For years I drove past the Smith plant on Capitol Drive every day. Completed frames were stacked outside, covered with snow and ice in the winter. Probably rusty before they arrived at the assembly plant.