[Arran]

You are exactly right-The scotts mostly prior to 1937 were not undercoated. That is the secret to a good chrome plating. I used to work in a factory that plated chrome cylinder rods and we used a nickel undercoating.

The late 1930's Scotts seem to be much better in their plating but I don't know if they changed their process or what. Maybe a new supplier perhaps.

That sounds like some sort of hard chroming process, was it a factory that manufactured or rebuilt hydraulic cylinders? There is a plater in the next city up the highway from me and they specialize in hard chroming, mostly for rebuilding hydraulic cylinders for the logging and mining industries.
In any event I don't know whether they improved after a certain date or not, one well pitted example I saw was a Scott Phantom chassis which was from 1938-39 or newer. The ones my friend had in Alberta were mostly Philharmonics, I think he may have had an All Wave 15 as well, the Phantom belonged to him. It's one thing you definately have to watch for on a Scott chassis, unless you get it really cheap it's a good idea to pay a little more and get an example with the best chrome you can.
It still blows my mind that outfits like Wells Gardner and Gillfillan (on the Western Air Patrol sets) seemed to use better chrome then Scott did even though they were sold for a fraction of the price. It really shows that slick marketing and a high price do not necessarily mean that you got an all around higher quality product.
Regards
Arran