20mm Oerlikon "N. F." ink stamping

I have a couple of WW2 US 20x110RB Oerlikon cases with the ink stampings “LOT N. F. 156” and “LOT N. F. 1567”. Was “N. F.” the code of the filling plant? If so, what was the plant called and where was it? Thanks in advance for any info. These may be contract rounds for the British Navy as one has a purple primer annulus (ball), and one has a blue annulus (incendiary). Could they have been supplied as primed cases and loaded in the UK?

Headstampa are:

A.S.C. 1942 20 MM-MK-II (LOT N. F. 156)

1942 20-M.M.-MK.-II (LOT N. F. 1567)

Thanks in advance for any info.