It would appear that people are still confused over this 1939 thing!

1. It was
not a census!

I do wish people would stop calling it the "1939 census"!
That just confuses others as to what to expect.
2. It was
not arranged by Registration District!

It was arranged by Local Councils (as they existed in 1939) - Urban District Councils, Rural District Councils, Metropolitan Borough Councils, County Borough, etc.
A different entity from Registration Districts!

My own family, in North Somerset, are to be found either in Portishead Urban District, or Long Ashton Rural District. Normally I would never search "Long Ashton"!