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NOTAM is real.
At present there are only 2 full time FISOs. The third was also the airfield manager, and he has got another job elsewhere.
As the two full-timers are on a five-day week, the sixth day is being covered by two part-timers (with 3 full-timers there was a rota that covered the Tower all week). The seventh day is the Monday. Having not been there for a while, I'm unsure of the reasons for de-licencing on a Monday but I guess it's because it's always been quietest day and they aren't able to provide the necessary fire cover due to using only one part-timer in the Tower (the second was always doubling as fire/ relief FISO).
There is a plan to go back to 7 days when Mr Bletsoe Brown opens his shiny new flying school. Don't hold your breath however, as it appears that could be a while away.
Of course Sibson - the new home of NSOF - is licenced on a Monday and has lovely smooth grass and great food....