The stamp is supposed to be perf 11 x 10.5, I have measured several times and compared to other stamps but the horizontal perf is 11.5. I can find no 11.5 stamp in my book. Help... I have a regular and the questionable stamp in the picture.
I think Don is correct .... perf 11 horizontally. One of the painful lessons in my early collecting days was learning that the Scott catalogue rounds perf measurements ... I think to the nearest 0.25. As I recall, the flat plate version of this design does measure 11 horizontally, but the rotary measures about 11.25 ... which Scott reports as 11. Why they would round down rather than up I do not know. Maybe an accurate measurement is closer to 11.2? In any case, I don't think it is actually 11. For future sanity, you might want to invest in a better gauge. I just measured some of the 11x10.5 stamps in my collection ... they all seem to be 11.25x10.5. Added: Just measured my 11x10 rotaries ... got 10.75x9.75. Added: In case you were wondering, I use the Scott Multi-Gauge. It has a variable scale that can measure perfs in increments of 0.1 and 0.25.