Erfurt Magazine release
 

I have been noticing that some Erfurts have the inspector stamp on the right side of the magazine release and some do not. Were they all stamped on this part? If you have a pistol that is not marked this way, does it indicate that the release was replaced? Could not find any info on this.

Yes, Erfurt marked all of the mag releases with a crown/Fraktur letter as required by the 1910 military directive. If unmarked it is almost certainly a replacement. The one except that Erfurt made was after 1914 when they stopped marking the grip screws.