Information regarding an NF call - 6 received - mentioned in the diary of the 165 Brigade RFA
Entry concerning NF call.
A pilot would broadcast an 'NF' call. See link: https://www.strategypage.com/militaryforums/4-808.aspx#startofcomments
Part of article: …”If a pilot saw one firing he would broadcast a 'NF' (now firing) call giving the zone (eg QB) and the HB number, the batteries allocated to that zone would then engage by looking up the target coords on their HB list. There was a system of dividing a zone into tenths and possibly hundredths but its not clear if this was much used by aircraft against opportunity targets…” (2nd paragraph line Eight).
Diary Page Entry for the 11/11/1917 - link: https://talk.operationwardiary.org/#/subjects/AWD0003yot