Trif;
Hi!
Sounds like you actually did the "right" steps on your zimm... it's really more that things don't show as well in pics as we might hope!
About the sleaves on the cable-ends - the only part(s) which I think you may want to address are those sleaves at the cable-end before the eyes - not the cable itself, which looks very very real and nice just the way it is! So, no, I am not suggesting that you repaint your whole cable - only do some messing-up of those sleaves, which look like new bare metal.
I am unsure indeed about whether those may have been made of any sort of "rostfrei" or non-rusting steel - and very very unlikely to be made out of aluminum. And not much else "non-rusting" would have been available for these things. Remember that these are heavy tow cables, so aluminum simply wouldn't cut it. Any form of "stainless" or non-rusting steel alloy was pretty expensive then as now - so probably not used in this rather basic and plebian application, I would think. But I am wholly prepared to be wrong!
In any case, perhaps most of these as seen in period pics appear to have been painted - either some industrial black enamel or over-painted with the regulation vehicle base-color. And of course they could be chipped and oh so dirty.
Hence my suggestion! "Toning down" the clean bare metal look would, I think, be a good step - whether it be simply smudgy dirt or some combo of "paint" and dirt or whatever.
Again, it's a nice build!
Bob