Gadget
The guidelines do suggest it should be n.k. (N/K as you suggested) in Miscellaneous instructions for clerks regarding revision of schedules.
https://www.rootschat.com/links/01thy/And amongst that folder is a 3 page reminder of the Objects of the census
https://www.rootschat.com/links/01thz/a lengthy version of the general UK census National Archive comment
"The object of the census was not to obtain detailed information about individuals, but to provide information about the population as a whole; listing everyone by name, wherever they happened to be on a single night, was the most efficient way to count everybody once, and nobody twice."
hence the comment Columns (a) and (b) name and relationship to head - the particulars entered into these columns will not require revision. They didn't use them.
