Hi Mike,
I have four McGraw (no cap) PD 150's and none of them have that mark but I do have two Cicotte 150's that have a similar mark.
img745.jpg
The Cicotte mark continues onto a Seymour PD 150.
Seymour%20_5_ - Copy.jpg
Seymour [5] - Copy - Copy.jpg
Having this mark on Cicotte/Seymour makes sense considering they are also
along side each other on one of the PD 150 plate scratch sheets.
Sheet%20F-B%20Copy.jpg