I looked and I looked, but...  

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Reitveld I think, with nice thick blanket stitching from memory.

Goodness, I feel like that sh...
Goodness, I feel like that should have been easier to find. Thank you very much!

Yes, it's
Rietveldt -- from 1935. The reference I have mentions "visible diagonal stitching" to the wool upholstery.
Here Rietveldt continues with the exploration of the dialog between the verticals and horizontals of architectural construction -- and, indeed, of Cartesian geometries in general -- and the diagonals dictated by the function of the object, namely a comfortable chair, a dialog which began (for him) in 1918 with the Red and Blue chair.
The architect returned to this form, with development, in 1958, in a commission for a new press room at UNESCO Paris.

After looking into it a bit,...
After looking into it a bit, would anyone know of any notable differences between the Metz chair and the Cassina chair? It seems leather is a Cassina-specific addition to the chair and earlier pieces were upholstered in fabric, does that sound right?

A great chair and originally...
A great chair and originally in a fabric upholstery according to the link below. One of my favorite chairs, but I like most of
Rietveld's furniture and lighting designs. I just checked the Cassina
website and the chair is available in fabric and leather, but I
think you are right in thinking that the leather chair was perhaps
introduced by Cassina.
I was only aware of the fabric option with the contrasting stitching, which is by far the most interesting and most authentic looking chair.
http://www.1stdibs.com/furniture/seating/lounge-chairs/pair-of-gerrit-th...

Utrecht, those chairs could Utrecht.
Utrecht is the name of this chair. Utrecht is a city in the Netherlands that also has the Rietveld huis (house).
http://centraalmuseum.nl/en/visit/locations/rietveld-schroder-house/