Looking in my collection (many hundreds of frogs) it is a fackt, that some markes made cooper/brass until 1937. Police issue used Alu und copper after 1941.
alu up to 1935 is not correct. the most of copper/brass from 1933-35 is ok... but a minorty , most found on Luftwaffe marked frogs are found until 1937.
mixed materials are common since 1940/41... the rest of Alu quantits had been used.
my lastest frog is marked 1945 and used metal rivets.
