Quote:
I thought that this stamp could be perforation 12 at the top and 11 under.
By taking a copy of the horizontal perfs from the top, rotating it 90 degrees and placing it next to the vertical perfs, we can see they are the same size.

Since plate number #10354 only was issued for Scott number 482 (imperf) and 499 (perf 12), it would appear to be a 499.
Don