Finally, finally, I've heard from someone with direct knowledge of the Lewis & Skinner company.
A huge thanks to Bill Luke, former signwriter and now artist (http://www.williamhluke.com/), for calling me the other day. Here's Bill painting a sign:
Speaking on the phone today, Bill had some great and colourful stories about Lewis and Skinner's manager Wally Parker and his reputation as "the biggest bastard in the business". Other folks at L&S include the following, complete with awesome nicknames:
It turns out I actually have the letter authorising Wally's promotion to L&S manager in 1951: