As previously stated, the Large Late war Font CNX marking is not all that common to find, as well as the Chain is a rare variation. This kit and component was in excellent condition at that. Which isn't always easy to find.
Here are some photo's of my kit which condition is not even close to the one that sold.
Certainly condition was a factor along with the story it came from a long-time collection. However, IMHO the RGK was the real prize and what likely drove the price. My experience has been similar to mrfarb's in that I have had at least five of the large CNX marked kits. Most had the higher-gloss paint and earlier pattern tins. The one pictured on the left and separate photo has the additional reinforcement rib and have found it to be the scarcer of the two.
From my observations over many years including spread sheets with several thousand cans and their contents recorded the CNX cans are certainly uncommon if not rare. As an example they are observed in far fewer numbers than the tan painted rco45, rco44, and/or arr4 cans. The late war chain-link RGK variant with the kit is extremely rare.
Again from observations, I am of the opinion that the CNX cans with the additional reinforcement ridge were made by Hawig. The cans lacking this ridge were produced by Mundlos. Cans from both Hawig and Mundlos were provided to G. Appel for finishing late in the war. The kits were completed and marked CNX some with SWE WaA20, some without. As noted by pwcosol the CNX cans can be found in several colors of paint and finishes. CNX cans without the additional reinforcement ridge are more common than those with this feature. The same may be said of late Hawig cans; most do not have the additional ridge although it is occasionally observed on late (cmr44 and 45) examples.