Jeff, thanks for the additional information.
The barrel is definitely a Simson replacement barrel. In the absence of a PTV/eagle or TP/eagle, it seems likely that the barrel was replaced by Simson sometime in the mid-20s.
You are correct about the suffix. The n-suffix 1920 DWMs are a bit of a puzzle since this letter was very much out of sequence (ns, a, b, n). However, there is ample evidence that DWM produced a full series of these in 1920.
The part of the mag safety that has been clipped off is the piece that rotated behind the trigger to prevent it being pulled when the mag was removed. These proved to be a bad idea and nearly all of them were deactivated this way or completely removed. Your gun still has all but the trigger block.
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