Upvote:5
According to this thread on another forum, the 75 DE C is for the gun model (75mm de campagne/field gun), A RS is probably for the manufacturer (Ateliers de construction de Rennes, according to this answer), 1181 is a lot number and 16 is the production year (1916).
Note that these are all very weak sources, but you might find better ones here or here.