A lot of seven series chairs have passed through my hands over the years - but these are new to me. They're upholstered in black vinyl and have all the correct proportions and details to other early chairs I've had - except in one detail - the legs are solid. They are nickel plated and all the rest of the hardware is typical of the period. Anyone able to enlighten me? 
anearlyone,
Have you considered asking Fritz Hansen (anyone) to enlighten you? Try the manufacturer first and ask all others second. This is how the "process of elimination" works or in this case it is known as "common sense".
You photos are poor at best. Very little may be discerned from these amateurish images.
Au contraire, niceguy. I can discern quite a bit from those photos. And that is that the upholstery looks very old given that the cambric is woven fabric as opposed to non-woven which is what is what is in use today. And that the upholstery is not a high quality job at all. The staples are not close together, the margins are not even on the sides, and the raw edges of the vinyl were not trimmed back to the staples (you can see them forming rippled bumps under the cambric).
In addition to that, there is no cover over the hub where the legs meet under the seat. I know later chairs had plastic covers which sometimes got cracked, but I found one early one that had a metal cover.
Were these ever upholstered in vinyl? I know they were done in leather.
Maybe I'm wrong, but I'm guessing that this is an old knockoff with correct proportions but other major differences, including solid legs. On the other hand, maybe it is a rare solid-legged prototype worth $30,000 that a hack upholsterer covered in vinyl at the behest of the owner in the 60s who kept sliding off the seat after a few too many schnapps.
PS I think that contacting the manufacturer is a great idea, but so is posting a query here because it adds to the huge and useful base of information that this forum is.
Oops, meant to add too that though I haven't seen a whole lot of old Jacobsen dining chairs in person where I turned them over and studied every detail, I am very, very familiar with upholstery methods and quality level in furniture from other Danish designers at this level. When they used vinyl or leather, the staples were nearly end to end and the vinyl was trimmed to about 1/8" from the staples. The work is extremely neat. When cambric WAS used, it was just as neatly done.
On better quality Danish chairs done in vinyl or leather, the staples are closer to the edge of the seat, making it possible to not have those big tucks in the material that you can see on your chair.
You don't show the back of your chair---how is it done? With welt or nailheads? That would be another clue---though again, all of this could have been done by someone after the chairs were manufactured. I don't think that's likely though, given the lack of hub cover and the solid legs.
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