German proof marks ?

Is there anyone who knows more than me about German proof marks ?
Probably from suhl

There are 2 different merkel shotguns and i will buy 1
But the proofs are very different and i expect the ages are also very different.

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Have got a book on german proof marks here that you can have free.

So i wonder what the difference is between a " smokeless powder proof " and a " nitro powder proof " ??