It would have to have been made after the US withdrawal of forces and the creation of the four occupation zones in the summer of 1945, which had been set out in the Potsdam Conference. And it would have been before the creation of a unified West Germany in 1949, from the merged US, French and British occupation zones. In theory, at least: once the stamping dies had been made, it's quite possible they keep cranking out harmonicas with that mark for a while longer.