Nice write up!
One thing I'd like to add to your data is that in 1938 there were two different trim versions of the Hudson.
One has the blacked out hand railings and piping along the bottom (as correctly came with your 4 chrome cars in set #20)
The other version has copper piping and brass railing trim (Not "blacked out") and came with 4 red passenger cars, in a slightly cheaper set #17 ($6.00 less for red cars instead of chrome).
You can also see in the catalog picture that both versions are shown. The trim difference is mentioned under the description for set #17.
This image is from the dealer catalog, fyi. The consumer version does not mention it, but does show it in illustrations. Hard to say if Gilbert was religious about making sure the correctly trimmed loco was always put into the correct set or not, but both versions of this locomotive exist and I have seen examples of both sets that are catalog correct.
