This one just came in today. Except for the blind perfs, yours looks a lot like this one. Yours actually looks more like 632 than this one. I might actually send this one in to be certified, it's just too close.
imodius: Save your money. Both of your stamps are 632. As to the one you consider a possible 596, the first thing to note is that the horizontal & vertical perf gauges are different. I simply measured the distance across the top that encompassed 11 perforations ... then measured the same distance vertically and found only 10 perforations. It is clearly perf 11 across the top ... so it must be 11x10 (578) or 11x10.5 (632). Made a quick comparison with known 578 & 632 ... and the result, not surprisingly, is 632. Added: Scott tends to round/truncate perforation gauges to the nearest 0.5 whenever doing so will not result in ID confusion. A close measurement of the 11x10.5 series yields a more accurate 11.25x10.5. In a similar vein, the 11x10 (578-579) is actually 10.75x9.75. On the other hand, the perf 11 (551-573) really is 11 in both dimensions.