I think you will have to go with the limited info in Senich's book.
As far as the STG rifle goes, there aren't that many examples out there to be examined. Not many people here on the parabellum forum are going to know the answers to your questions, as they collect P-08's, not STG rifles, (a far more expensive hobby.) I would actually post these questions on the Sturmghewer forums, where you may get more people who are knowledgeable in this subject.
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