That's correct, they were not painted originally from the factory. It's fairly common to see those wheels getting rust on them from being exposed to the elements too much. If it does not clean up well with 0000 steel wool, you can paint them with an automotive paint. Prepare just like you would the body on your car for painting. We use PPG Liquid Metal for refinishing the spokes on all of the wheels with the satin chrome. It's not cheap ($130 per pint) but it is the closest thing you'll find to that original satin chrome look. However, there are a lot of the 72-74 wheels available for less than $100 in really nice condition.
Dave