my Polish Grandfather in Hitler Youth? HOW?

Not much, since such things are hard to prove. Looking right and being willing helped.

Can anyone tell me more about this document ?

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That only proves that Hitler considered Polelock equal enough to Germans to join the Boy Scouts

My Polish grandfather had the same forms and was forced to join the Nazi Youth at age 14. He lived in Warsaw, prior to 1939. Before part of his family escaped to the USA. (Where is joined the army and served in army.)

If your Polish grandfather had the same document as above (which is an identity card) and joined the Hitlerjugend, then he was an ethnic German/ Austrian. Wonder why part of his family had to flee to the US then...