Rslny, first is the mark in red at the left pointing at the dot in the oval frame. This dot is on all plate 11 and 12 stamps. The line at the right indicates that it is a 10th column position of the left pane. The full plume from the stamp above (orange arrow) indicates that is can only be plate 4 or plate 11 or 12. Plate 11 is scarce so plate 12 is assumed. The fact that the full plume ("C" relief) is above a "C" relief (in 10th column) stamp, indicates that this stamp is a bottom row stamp. Therefore, the stamp is plate 12, 10th column, bottom row = 100L12. The pink arrows are plating marks.

ps: did not mean to rehash what txstamp posted as I did not see it before I posted.