I have just finished sorting my large queens. I have been searching the forum and found this post. This post is about the 6c so I hope you dont mind me jumping in.
I have spent much time searching, but have not seen a similar stamp yet.
Plate 1. Perf 12-66. Doubling on the right side, including the scroll, oval and frame lines. It also has a vertical mark in the R6, from ball to below. Could this be flaw #7 in the large queen report by Hans Reiche ?. Not in great condition, but I have much worse.



