Second World War Luftwaffe Flying Boots.

A query here for the Luftwaffe experten amongst you…! I have a pair of the early-war twin-zip boots. I know the later boots were reduced to a single zip for economy, and later still had heating elements introduced. Mine are twin-zip with no heating. Does anyone have any idea of the dates these started to be made and when they were phased-out for the single-zip version….? I’m guessing the introduction was mid to late 1930’s and the change to single-zip perhaps 1941- ’42, but those are just guesses.
Any kosher information gratefully received.

S.