Without reservation, the stamp of the "G" ore tub in the shape of a sole "M", probably a process mark noting the amount of effort, is that of Gebrüder Merkel. The Merkel boys, Bernhard, Oskar, Gebrüder were heavily sourced by Heinrich Scherping.

m-4's Heinrich Scherping sidelock

Heinrich Scherping in above post.

Merkel drilling but I don't recall right off where this pic originated.

Gebrüder Merkel's typical stamp.
I'm going out on a limb and a step further and say that the following is also a Merkel process stamp:



on a E. Schmidt & Habermann drilling. I invite any and all criticism to prove otherwise.
Kind Regards,
Raimey
rse