Could the cases be pre grooved/creased in some way so that they follow the shape of the creases when outside pressure is applied,or were they swaged in by some sort of machine similar to a hydraulic pipe swager,I would assume that part filled with sand and sealed and pressure exerted on the outside of the case in certain areas with the use of shaped dies would lead to uniform creasing/fluting,would heating the brass almost red make the flutes easier to form with the brass being made soft,I would imaging some of the elaborate "trench art"was made in workshops and the more simpler versions made "at the front",I have a WW1 18lbr,case that has been bulged outwards to form a sort of trophy cup,the handles are made of driving band,the brass is beaten very thin and has cracked in places over the years,
Cheers,
Don,