Mogens Kold?  

Possibly Rastad & Relling, it is very similar.
BTW tchp, I have an interiors magazine from 1960 which has an advert from J L Moller showing the Model 71 designed by Arne Hovmand Olsen.

Thanks
If Arne Hovmand Olsen is also cited as the designer of the Moller #71 chair in a 60's ad, in addition to the citation in the 50's Moller Catalog, I suppose that should be proof enough to satisfy anyone that he designed the chair, and not Neils Moller. If you ever have a chance to post an image of the ad, I would love to see it.
The frame of Rastad & Relling chair you posted is very similar - especially the front rail of the seat, so it would not be hard to believe that they could be the maker.

Notice that
each of the chairs above has its own back design. Similar, but recognizably different . . . "There's nothing new under the sun -- or, Great minds think alike" ?

There is no doubt for me...
There is no doubt for me that the Model 71 was designed by Arne Hovmand Olsen.
I can understand how over a long period of time the wrong designers name can become associated with an item.
What I find interesting is the fact that J L Moller now catalogue this design incorrectly, even though their archive literature lists the design as Olsen.

Thanks for posting that. It...
Thanks for posting that. It is a great looking ad - and with Olsen again named as the designer of the chair. It is a peculiar thing that the manufacturer no longer makes any mention of him anymore.

Another old Moller ad, citing...
Yet another old Moller ad, citing Hovmand Olsen as the designer. There really can be no doubt that the Model 71 Moller chair is a Hovmand Olsen design.

it often surprises me
that firms lose interest or expertise in their own history.
the same with archives, wich get lost, dumped or even burned...
within the process of doing business and selling products, history or what happened in the past seems less important.