I have just rediscovered the 1939 England & Wales Register. I had previously come across it but not realised its potential as a source of information. Indeed I was puzzled and discouraged by the number of redactions in it.
I think Ancestry.com best explains how it works –

Naturally I could not resist looking up my parents in this register –

The three redactions are of course my three older siblings – of those the oldest is 86 so there is a bit of a wait before they are visible to the public – myself and my next oldest sister were not yet born in 1939. so no black line for us.

Who says genealogy is boring?