It is possible that there were some amplifiers scattered about, but I doubt this. Usually the speakers are all in parallel, switched by the relays. I expect that the in-wall volume controls are generic speaker level controls. And, I expect that those DIN plugs are a variant of DIN Speaker plugs -- there will be a larger flat pin and smaller round pin in the central area. The outlying pins are communication and power distribution and the central pins are the speaker connections. There are two plugs, left and right.

Usually, these systems were daisy chain, not home run.

Pull a volume control and trace its feed. You'll be able to tone the speakers and locate where the relay box was or is located. The relay boxes were terminated with raw wire.