Ken Wallace wrote: I have a Willingale with what appear to be the same ends.
Percy Stallard designed very similar dropouts to use on his machines, and also sold them to other frame builders. All the Claud Butler ones I have or have seen are stamped CB.
There is a photo of a Percy Stallard machine at:
http://www.localhistory.scit.wlv.ac.uk/
Scroll down to the bottom of the page.
Peter Brown. Lincolnshire, England