Wow, some fella back in the early '20's went to a lot of trouble to build this, and did a heck of a good job for a home brew, labels, squared-off wiring and all.
I think what I would do to get a grip on it is sit down and start drawing the schematic. I've done that with orphan sets. Your first sketch will be full of erasures and lines scratched out. Then you copy it over neatly and double check against the radio. Then what I've done (since I like doing it) is make the final copy using a drawing program like Paint. Once you have a schematic it should explain itself.
Reece