This is a May 69 magazine ad, note the early tow hub design- much like I mentioned about the early TB30 design.
Here is a March 1973 catalog of the later hub design.
I'm really beginning to think there is no timeline that corresponds with color. It would help if I could figure out when the timeline began.
Now here is a 1974 Midwest publication offering NMW products where it seems they used some of the earlier art work for the ad.
Here are both the later designs in both Silver AND Orange. It does look like ornage paint here, not primer but if you look at the thread, there does seem to be a variety of orange.
There is a trick to when you mount these onto your tow wheels to save a step at the track. Anyone have a picture of some wheels modified for these tow hubs? Not required to be used but when you see it, you'll get it.