Why not many Catenary Group Chairs for sale?  

Hmm
I don't know, but there is a table on eBay right now...

Wrong top?
Thought the original table was square.

It SHOULD be square!
My guess as to why there's so few for sale is that they sold to executive offices and to lobbies, much like the Mies Barcelona chairs and tables.
I also don't think they sold particularly well...nor did Nelson's swell but rare Black-edged Cube Group of the same period,.,

you know...
the cube group is still available in Herman Miller Asia...strange eh?

I was just looking at Herman Miller Asia's website
and I did not see this Cube Group.
Can you post a direct link?

OK...found it only at the Japanese website
see above

sorry...its japan... http://www.hermanmiller.co.jp/en/product/nelson_seating.html
they just launched the lafonda chairs too...which im sure they are getting from Vitra...but the Cube collection is weird because they actually have to have someone contracted out to manufacture it, and it's not available at any other part of the world...so strange. they have an old nelson desk as well available.