Date of Niels O. Moeller stamps  

That is a post about 1971 Møller stamp. The earlier mark (roughly during the 1960s) is a medallion, and sometimes a JLM ink stamp also. And even earlier than that (roughtly during the 1950s) Møller pieces tended not to be marked at all.
I don't know if this will actually tell you whether the Rosewood is Brazilian or East Indian though. Where did you see that information?

I found this mention of it:
"About 1970, timber agents in Denmark stopped importing rosewood from Brazil and replaced this source with rosewood from India ",
under the following link:
http://www.paeredansk.com/n-o-moller-1966-carver-vintage-1960s-product-p...

I emailed pictures of furniture and stamps yesterday to a specialist dealer who tells me the chairs are East Indian Rosewood - although he didn't say on what basis he believed this to be the case. I can make further enquiries with him about this.

I don't think I would categorically believe that all timber agents in Denmark stopped importing Briazilian rosewood in 1970. In 1992 Brazilian rosewood was banned, and even after that, I doubt I would believe that absolutely every timber importer stopped importation. There is, unfortunately, a lot of demand for Brazilian rosewood for making musical instruments, even at extraordinarily high prices. I would believe that Brazilian rosewood get expensive enough to that Møller stopped using it long before 1992, just because of the cost.. That website has some interesting information that I tend to believe is reliable, though.
You may be able to determine whether you have East Indian rosewood yourself by looking at the density of the pores, perhaps at the tops of the legs:
http://www.wood-database.com/wood-articles/distinguishing-brazilian-rose...

here's some pics of the end grain with measurements for scale, if that helps-
(lots more pics at http://www.hobbithouseinc.com/personal/woodpics/#letterR)

Thanks leif that's very helpful. I have also heard now that the 'black lines' in Brazilian Rosewood are thicker. I am now told also by a dealer that the chairs are a mix of Rosewoods - Brazilian and Indonesian - so this is even more intriguing!

I've never heard of Indonesian Rosewood, so, if that is not a typo, then I would be suspicious of this dealer's information. I somehow doubt that Møller would use different rosewoods such that a dealer would be able to tell the difference in casual photos. Rosewoods are famous for their varied appearance such that it can occasionally be hard to tell which rosewood board is which. I think that Møller would have guaranteed a certain uniformity in a chair, so that even if it were composed of different species it would be nearly impossible to tell.

I believe Indonesian Rosewood is also known as East Indian, although this may be my misunderstanding. The dealer was local and came to look at the chairs in person. He says he has been to the Moller factory in Denmark and that they would just make use of the pieces available.