What is this artistic motif called - and during what time period was it popular on US tokens?

It is most commonly called a propeller and comes in many slightly different forms sometimes with different names to differentiate the devices. You can find them on early 20th Century tokens and in the catalogs from some of the major regional manufacturers of the time. Most state catalogs that get down to describing the ornamentation on trade tokens use propeller as a mainstay of their terminology.

I would say he most prolific use is post-World War II as propellers were very common on the thousands of tokens struck by Meyer & Wenthe, Schaaf, Osborne, Northwest, Wendell's, etc.. It seems like every parking token ever struck has a propeller or two on it.