Die-cut (stamped) triggers were indeed manufactured during WWII. All pre-WWII triggers were milled.

Stamped triggers can be identified by the semi-circular profile of the thumbpiece. Milled triggers have a slight "bump" on the tip and a slight depression at the top. Stamped triggers have a plain semi-circular thumbpiece with no irregularities. Stamped triggers also quite frequently exhibit shear marks along the edges.

Hope this helps.

J.B.