Ya, this was a 'joint issue'. Similar, or identical stamp designs issued by both US and Switzerland. Here's the US issue of the same design:
Joint issues would make an interesting topical collection! Many times, you could buy both countries' issues from one postal authority. I don't think this particular Swiss issue was offered by USPS, but I might be mistaken...
This happens every once in a while to me - I'm researching something and trying to prove a negative. I suspected as much but couldn't find the page where it said, "You cannot put this Helvetia 160 Hans Hartmann Courvoisier stamp on an envelope for US domestic postage."
Finally I thought, "Hold on.......StampoRama!"
Sheepshanks - just curious - am I not seeing a note?