On this rim transition question. I have had a look at my old cycling catalogues amongst which I found a 1939 Claud Butler brochure. This shows that his models for that year were about 50/50 between 26" and 27" rim choice. If anything probably a little more supplied with 26". The tandems were all 26". Interestingly the "Massed Start" which I believe was his first welded frame which he first produced in 1938 was fitted with 27 x 1 1/4 rims. This frame was also the first to be fitted with the then new CB Continental ends. Have also looked in The Complete Cyclist 1949 by R.J.Way and Cycling Handbook 1950 by A.L.Pullen and these both state the most common British size is 26" but for lightweights and for racing the 27in. size has definitely taken first place. It would appear the fashion was turning in 38/39 towards 27in. rims and the war got in the way and delayed the trend.
Best wishes and be lucky. Michael Butler Huntingdon UK.