The proper identifications are:
F N - Fabrique Nationale d’Armes de Guerre, Herstal, Belgium - much of the 7.9 ammunition of this era from FN was made for Lithuania, with this headstamp style. I cannot say that all of it was. I have a similar headstamp but instead of the lot number (month?) it has the well-known Lithuanian “Pillars of Gediminius” crest on the headstamp, the same crest that appears on the front receiver ring of FN-made 98 Mauser rifles and FN Browning GP (HP) pistols, on the top front of the slide, made for Lithuania.
CI - while some of this could have turned up in Korea, it is, in fact, a headstamp made in Germany for the Spanish Civil War. “CI” has been identified by the German Group as a product of Dynamit A.-G. vormals Alfred Nobel & Co., Werk Empelde, using the codes P120 and emp on ammunition made for the Wehrmacht in Germany.
AI is not, in this case, Artillerie Inrichtengen of Holland, but another clandestine Spanish Civil War headstamp, identfied in Germany as the mark of Deustche Waffen- und Munitionsfabriken A.-G., Berlin-Borsigwalde using the codes P131 and asb for ammunition made for the Wehrmacht, and “B DWM B” etc. on ammunition made commercially and for export.
A D reportedly stands for Artileri Dirbtuv