The German Youth knife were produced from 1933 until 1942 when they were discontinued by which time over 15 million HJ knifes had been produced.  Most of them can be classified into three types. The early once were produced between 1933 through 1936. Those are with thin blade, full makes marks on the blade, motto. Next is sort of a transitional period. (1936-1938). That is the time when German regulation required to use RZM designation. Those knifes will have slightly ticker blades, maker mark and the RZM designation together, motto. The late period from (1938 until 1942): RZM markings only, no motto, think blade.  It is important to know that there are many reproductions out there, perhaps most of all other German edged weapon. New buyers should be extremely careful and if not sure ask advise from one of the knowledgeable dealers out there. Any will be happy to help out and give an advice. Good luck collecting those blades!
Oh, by the way all Youth Knifes with markings "Germany" is the post war reproduction.