Thanks for all the help on the Claud Butler frame, which, per several responses, I now know was built in March 1950. The lug pattern exactly matches that for the Allrounder in a 1953 CB catalog and also the photos of a 1956 Allrounder, both these generously supplied by list members. Peter Naiman, who I bought it from, also now advises it is an Allrounder. I've not seen a 1950 catalog or photos of both 1950 Allrounder and 1950 Avant Coureur for a conclusive comparison, but unless CB used on the 1950 Avant Coureur a pattern that was used on the Allrounder in 1953 and later, which seems unlikely, then this must indeed be a March 1950 Allrounder. The collective knowledge of this list never ceases to amaze.
Regrads,
Jerry Moos
Houston, TX