Nice job finding reference copies.

I don't see a single stamp that exhibits the mark shown in Neinken, as drawn for 100L2. The nearby one on the OP stamp may be what he meant, but its not clear. It does appear to either be a very transient marking or a mistake in Neinken. Certainly not fully consistent. Good to know, thanks.
As an aside, Ashbrook (possibly Neinken as well) definitely suffered at times from doing plating drawings from black and white photos. There are plating marks drawn by Ashbrook on mats, which show plating marks that are really cancellations. Its harder to tell the difference with a B/W photo (vs color), what is a plate variety vs what is a cancel. Maybe that happened here, who knows. Its worth keeping on the lookout for this over time.