The 1939 Registration was the enumeration to enable the issue of ID cards, ration books etc. in event of the outbreak of war (Second World War).
The details enumerated were
1. Names.
2. Sex.
3. Age.
4. Occupation, profession, trade or employment.
5. Residence.
6. Condition as to marriage.
7. Membership of Naval, Military or Air Force Reserves or Auxiliary Forces or of Civil Defence Services or Reserves.
Some look on it as a census but it did not fall under the 1920 Census Act.
Cheers
Guy