Metal artillery slide rule Nicholl WW1 British army 1916 England

This large all metal slide rule is an early model from 1916 by J.A. Nicholl&Co from London, England. It is double sided and shows various calculation for artillery firing. It also features the arrow sign on rule and case indicating it was issued by the British War Office (now Ministry of Defence (MoD)), making this a first World War item. The name on the front is “APP OBS FIRE SLIDE RULE MK VI” and besides the name and year of maker it has the instructions engraved on the right that read “Minutes in apex angle cr battery range in yards”. There are also red dots on the from to help calculate. The entire rule is made of aluminum and has some signs of age. The end pieces and cursor are of a different material that patinated differently (possible a mixture with silver and copper).  The slide screw on top works fine but has become a little crooked somewhere in it’s life. The original leather stitched case has seen better days (half loose and missing a small part on the end) but has fulfilled it’s job beautifully keeping the rule in good shape. It measures ca 45 cm or 18 inches wide. Check the video on how to work your vintage man tool : slide rule operation instructions!