At first glance, it looks correct, but looking up closely, the profile of the top part of the chair back side view is different and the number of webbing on the seat and back is also different from the original, unless this is a later replacement.
The model 666 is Knoll's earliest chair dating from 1941 and comes in several variations. With the rationing and restrictions on materials for furniture making during WW II in the USA, the original webbing were rejected parachute straps. Plastic webbing and leather straps was offered on later examples and has the same width as the parachute webbing.
There are many books and websites out there that you can look to confirm. Hope this helps.
The design and construction of this chair is so easy to copy that other manufacturers made versions with slightly varying dimensions and profiles.
http://www.knoll.com/product/risom-side-chair
Was looking through my Esempi Tavoli book from 1950 (trying to find Leif's rosewood table) and came across these images. These chairs look similar to those in question and also use tacks to attach the webbing (but differently). The table is attributed to Neil Morris for Glasgow....but nothing about the chairs. Could be worth some additional research?
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