Have these scandinavian looking tub chairs been on here before..?  

psssst---spanky is a she. Just sayin'.
The fabric on the chair I redid is "Fluffy" by Luna and it's no longer made, sadly. But there are some similar fabrics out there. It is a boucle weave with some wool content-- very soft and wooly (much more so than Knoll "Classic Boucle"). I still have a yard or so that I'm hoarding.

Sorry Spanky for the un-intended gender issue 🙂
Thanks for your comment! Would you agree on the choice of fabric for this type of
chair, or do you have alternatives in mind?
I was thinking of a medium grey with the pillow one side in this grey, the other in dark grey (same fabric).

I think that type of fabric is a good choice. But if they were my chairs, I'd seriously consider doing them in Kvadrat "Tonus" which was used on a lot of curvy Danish chairs of that era. It's also just a beautiful fabric---but it's a flat weave, not a boucle, so it would be a different look than what you have there now.
I'd also skip the welt around the perimeter. I like the nearly seamless look of this style chair that you get when there's no welt. I'm also not so crazy about two colors for the cushion. I think it breaks up the line too much. An alternative would be to make two cushions for each chair---one in the same fabric as the chair and one in a contrasting color. Then you can easily change the look of the chair without interrupting the lines so much. This would mean getting an extra yard and a quarter of fabric, plus making an extra foam pad (if you don't want to mess with putting zippers in, which is tricky to do well on thin, rounded cushions, unless you don't care if they are reversible).
Just my opinion, of course.
This type of chair is a pretty simple upholstery job BUT it's not easy. You have to be meticulous about keeping ripples and tucks from forming, and the seam around the perimeter has to be a clean line with no dips or waviness (I mean overall waviness, not the even little waves that form when you do the ladder stitch correctly). You will also pretty much just have one chance to get the fabric on the inside back glued to the foam; the glue bonds very quickly and if you wait a few seconds too long to reposition the fabric, the foam will tear up with it. Make sure your upholsterer can handle this kind of job. I've seen a lot of curvy chairs where the upholsterer just stuck some button tufting on the inside back to hold the fabric down rather than gluing it.
Whatever fabric you choose, make sure it has a fair amount of give to it so that it will go around the curves, and make sure that spray adhesive does not soak through it. I actually had a problem with glue soaking through on that Luna fabric in just one little spot and had to redo the entire inside back.

Is there any chance the brass sabots are not original? Perhaps the owner wanted sabots (for mopping perhaps) and these were generic retrofits sabots that fit over these feet.
Sabots are very uncommon on Danish Modern furniture, as is well known, but I have never, ever, ever seen sabots that fit proud of the legs, like big wool socks. It is not a good look at all. And the legs are turned on a lathe to begin with, so it is trivial to recess the bottoms to fit the sabots, so it is not like it saved money in the making of the chair. I just don't get it, unless the sabots are not original to the design.

Well, Leif, the legs are turned a bit broader on the bottom to fit the sabots.
I know, it's weird but so it seems these are original. It's the only thing about
this chair which doesn't suit me...
Thanks Spanky for your extensive reply. I hadn't thought about Tonus,
but I do admire the tight upholstery on a lot of Juhl pieces, which seem to
feature this flat weave fabric?
Indeed, the welt has to go!