In the top right hand corner of each page of the 1939 register is a a pair of numbers recording the Registration District and sub-district for that page. As has been commented on in a previous question it is relatively easy to find the name of the district from the number using the 1931 census reports available at histpop.org.
However, there were many new districts created in the 1930s that don't appear in the 1931 census e.g. Ince in Lancashire and Wednesbury in Staffordshire. These generally appear in the 1939 register as a number of the district they were formed out of together with a letter, so Wednesbury is 371a formed out of the Registration district of West Bromwich (371).
My question is does anyone know of a definitive source of Registration district names mapped to numbers in 1939. I have asked both the GRO and FindMyPast to no avail.