I don't know about these specifically, but I have run into certain clubs in the past especially on ebay that may not be available in the United States but are available in other countries. Sometimes they are purchased overseas and brought back by the miltary, I'm not sure about these though.
Was that common place for Wilson back in the late 60s/early 70s?
I'm not sure about that time frame. It could be a case where they came out with that model, and it didn't sell well. Maybe that's why it's hard to find info on it. I've owned two sets of later model wilson's and liked them alot.