Schrage Musik
I think it took a very long time for the effects of schrage musik to filter through, because it was so effective. The crews that encountered it simply didn't come back to tell the tale. The few that might would have not seen their attacker - which would not have been unusual and which would not have aroused suspicion that a new weapon was being used.
Schrage Musik was around earlier than some records suggest. A few nightfighters were unofficially modified before the official policy came into effect. A relative of mine was killed in a Wellington in Jan 1942; one of three shot down in a period of forty minutes by the same Me110 of NJG2 from Leuwaarden. There is evidnce to suggest they were bought down with Schrage Music, but with a twin MG42 installation and not cannon.