Møller 75 chairs  

Doesn't sound original
to me, but who knows. Could you post a closeup of the seat material and one of the underside of the rails?
Peoples' memories aren't terribly reliable. Maybe the first owner had a cat who destroyed the seats early on and they were redone with something the cat couldn't wreck. And then the person from whom you purchased the chairs had forgotten this since it was so long ago, or didn't know about it. That's one explanation I can think of, if the material isn't original.

Close-up
Hi,
Thanks for your reply. I've attached a photo as requested. It is the frayed edges at the end of the seating material that struck me as a little odd.

Well, I can only see the one
of the underside but it does indeed look like it could be original! How odd. It has the same diameter, degree of twist, and color as paper cord. It seems very unlikely that someone redoing the chairs could have found a material this close a match in appearance to paper cord. Did Møller offer this material for a short while, maybe? I have no idea!

Might be original. Something...
Might be original. Something about the loops on the nails looks a bit sloppy. Also the medallion means that is an earlier 50s or 60s chair, and I would think that plastic cord is later. But all this is very subjective.
JL Møller still exists and has been known to respond to this sort of question. Send them some photos and let us know what they say.

Iv'e had chairs in the past that had a plastic/Nylon type cord. At the time I thought it wasn't original but now I'm not so sure.
I also had a chair which looked like the normal paper cord but upon close inspection you could see the paper had worn to reveal a nylon core.

Thanks.
Thanks for your useful comments and suggestions. I'll contact JL Møller (my curiosity is getting the better of me!). If they respond, I'll post their reply here.
Cheers.