David, thanks for posting those pictures, you've made my day !
You're absolutely correct in suggesting that the machines are Nortons (Is it the same bike in both pictures ?)
Based on the assumption that census numbers followed frame numbers which is a little dubious on these early machines but the only option in the absence of surviving records, C4102632 would have been frame W4231. I am rebuilding W4216 as C4102617. I don't think that I'm going to find a closer match for detail info
The machines look standard and I hope that the pictures date from early 1940. I'm particularly interested in the continued use of the pre-war style black background for the census number with what look like hand-painted ciphers.
It looks as if I am about to become a customer of "Stilltime". Do you happen to have an image reference for the close-up shot ? I can't find that one on the web site.
Rich.