Borge Mogensen chair ... or not?  

Aesthetically, I would say not Mogensen. The whole design is in sufficiently minimized; the are far too many oddities for the eyes to fixate upon. Mogensen designs are very often the cleanest of the clean.
And there are little details like the backrest outside the arms with the exposed screw holes. The lower sling going front to back leaving you legs to fight with the front cross bar.
(On a side note, has it been put together wrong such that the Danish Furniture Control mark is visible from the top?)
Finally, I think this is the most objective point: Fredericia Stolefabrik/Furniture is the most likely theoretical manufacturer-- if it were Mogensen -- but Fredericia was not part of the Danish Furniture Control organization. FDB Møbler was also not part of the Danish Furniture Control. And Erhard Rasmussen was not. So If it wasn't made by a Mogensen related maker, it can't be Mogensen.

Hard to argue with Leif on this one. As an aside, there is a new Mogensen book that looks interesting.

Funny you mention a new Mogensen book, jesgord, as I just got my hands on an old one from 1968.
The OP's chair is not in there.

And a nice book that is too!

The old book has been on my wishlist for a long time. How is it?

Good so far from just skimming through. Appears to be more of an overview of history and philosophy rather than a complete catalog. There are also some photos of his house, furnished as you might expect. The dual language part is nice.

Hi Leif, Jesgord & Cdsilva,
Many thanks for your interesting coments.
Leif : my first impression was a Mogensen design mainly for 2 reasons :
1. The "feeling" of the materials as I already had some Mogensen chairs (Spanish, Hunting, ...) in the past - it seemed to me to be the same wood / leather type & quality when I touch or feel these.
2. Even if more "rough" or "less minimalistic" - I agree, I personaly found similar design details / "signature" details as the "C-form" cuttouts in the back leather on each side to leave space for the armrests (Spanish, Hunting) ; the way you can remove the leather back easily; the type of wood assembling in the "corners" bewteen amrrests & feet (2256 chair), the materials, etcetera... I immediately thought this could have been an early Mogensen work, but with my lack of documentation in Danish furniture... this is why you guys rock!
I admit I also did not know Fredericia Stolefabrik was not part of the Danish Furniture Control organization - this is a point, for sure ! Do you know if this was "forever" true? Would you point out another designer / manufacturer to help me ID this unusual chair?
Jesgord & Cdsilva, thank you for linking these books I didn't know, I will definitely try to get these to improve my knowledge of this great designer!
***To all : sorry for my english - thank you for your understanding***

Leif : look what I just found behind my Spanish chair : seems like Fredericia was finally part of the Danish Furniture Control or did I miss something? 🙂

Fantastic! You have proven me wrong. Fredericia was indeed part of the Danish Furniture Control.
Mogensen is certainly the best known for oak chairs with butt leather sling seats, but there are certainly plenty of other companies and designers who might have done such a thing. Poul Hundevad comes to mind. (Not that I think it is a Poul Hundevad either).
I still think it is very probably not Mogensen. It just does not look right.

Same story behind the Hunting...

No problem Leif, always a pleasure to read coments & to get other people's advice so thank you again for the coments & the Poul Hundevad track! I will check this now!! 🙂

Does anyone know if there is a sort of "bible of scandinavian design chairs" available? Like a "Repertorio" but for scandinavian furniture? 😉 Maybe I ask too much... but this would be a great addition to my bookshelf!

Have you tried digging through the danish furniture index? I would not get too precise with the query. You would probably have to look at every chair that comes up in the category "Chair" to know for certain that it isn't there.

I was thinking about this some more. And I think the Danish Furniture Control medallion might help you narrow the list of makers down.
For instance, Fredericia, who, as I shall remember for a long time, was part of the DFC, but I really have NEVER seen a metal medallion on Fredericia product. Only the paper tags. Other companies used heat brands, foil stickers, fabric stickers, etc.
And there are not that many makers I can think of who did use a medallion DFC mark.
So you might try to draw up a list like that, and then you can focus on whether the chair looks at all likely for the company's designers and product. For instance if they used a designer who likes sling chairs and/or oak and/or finger joints. With some work you might narrow it down to a small enough number of candidates that you can dedicate serious time to each.
As a final suggestion. I usually see these DFC medallions on pieces from the early 60s.