I was an electronics tech in a past life. They say education is what you have left after you forgot what you learned in school and that's what I have left. Anyway for some reason this fascinated me so I'll give my .02c which is worth every penny you paid.
The +12v is no doubt correct since I never heard of a DC radio in a 12v car system that used any other voltage. Your speaker impedance should be safe at 8 ohms. With that many transformers it is logical that two of them are audio couplers which are pretty forgiving of impedance matching.
The speaker wires look like green, violet black to me. Its a good bet that black is ground shared by both speakers and the green and violet are front and rear. Once again, with a transformer coupled output it is likely forgiving of wiring mistakes even if you wired front to back on one speaker. The mechanical preset selector I know nothing about except it looks old, dry and brittle. I would blow it out gently with air to get out the dust and then spray with electrical contact cleaner (you can spray that stuff anywhere and it flashes off clean, no residue) and put maybe a drop of light oil, (do they still make 3 in One?) on the moving parts to loosen it up? I wouldn't use any spray lube that got in the electronics.
My question is about the 7 pin DIN plug. Was there an optional 8 track or graphic equalizer available in 71? That's the only explanation I can think of.
Good luck with it. IMO it would be way cool to have a working original radio in the car!
Oh, you probably already know, but a coat hanger stuck in the antenna socket will work for bench testing if you don't have one handy.
Have fun!
Bernie
