Pierre Paulin: F598 Groovy Chair  

They're very different.
On the first-series chairs, the legs are concave and tapered, so the chair overhangs its narrow footprint.
The sides of the later chairs are like Senator Larry Craig's self-characterization: Straight, with a wide stance.
Here are some photos of a first-series chair:

(continued)
And here are photos of later chairs:

And...
This is obvious, but I guess I should mention it anyway: The first-series F598 has a wide groove between the seat and the back (hence the name "Groovy Chair"), but there's no such groove on the redesigned F598.
The original chair's wide groove emphasized the fact that it was composed of two parallel undulating ribbons; it was clear that the chair was a close relative of Paulin's beautiful Ribbon.
In the redesigned chair, though, the seat and back are pressed right up against each other, so they no longer look like two dramatic floating ribbons. Instead, they appear to be just one molded lump, with a seam on each side and an unnecessary smile-shaped cutout in the middle. In my opinion, it's a significantly weaker design.

"Senator Larry Craig's...
"Senator Larry Craig's self-characterization"
Ha Ha Ha Ha Ha Ha!!!

Thanks so much for your help!
Thanks so much for your help!

Question
Help!
I have the original chair. Wondering if Artifort was the manufacturer of the original? I can't find any markings.

yes artifort was the...
yes artifort was the original manifacturer... it was stated on a sticker on the inside part of the narrow leg. yours is probably gone or has been removed.

I confirm that M chairs have a sticker as per photo under link
See also more details of a chair renovated respecting all Artifort details (it is not a first line of M chair)
First editions had black base which was replaced later with aluminium, giving more contemporary look
http://www.fluxvintage.com/seats/artifort-groovy-chair-f598-orange-m-chair-vintage