EV607797,
The service was a Zinsco "all-in-one" with some (about 50%) GTE/Sylvania breakers in it. Zinsco was sold to GTE/Sylvania in 1973. I don't know any more than that.
There's lots of 100 Amp services here on the West Coast.
Zinsco......Thank goodness that cancer didn't spread much to the east coast! They existed here, but not for long. No offense intended about the 100A services, probably due to lower heating loads in your parts. We do see them here, but rarely on a residence that's less than 40 years old.
I will say that I've seen some THHN/THWN that was either black or white as far as the base insulation color and the nylon outer covering provided the actual coloring. I remember that red was usually white underneath the nylon. I think that I saw blue and green done the same way. I guess that manufacturers saved money by NOT adding pigment to the base conductor insulation and just coloring it on the outside.
There was a time when you could actually specify THHN or THWN, but not anymore. All you will ever see today is a combo. Maybe what you saw was THHN from the 1960's?
Sunlight will do quite a job on anything. The outer nylon jacket cracks, exposing the underlying base color.
