Short answer: Yes, you have a #610. Ignore the measurements in the catalogs.
Longer answer: Use methods other than the measurement of the design first. As you did, check the perforation rate first.
Also check the back side. The flat plate press sheet very often have minor set-off of black ink where the sheets were stacked. The rotary press stamps won't show this.
Additionally, the flat-plate plates were arranged differently than the rotary press plates. The flat-plate stamps had 19 of every 100 stamps with one or more straight edges. The rotary press stamps are all perforated on 4 sides. The straight edge along the bottom is a strong sign of flat plate printing (or a trimmed rotary print).
Lastly and seldom necessary,for checking the image size, because the difference is very small, many collectors use another stamp of known printing method as a template to compare height/width, rather than a ruler. These stamps were printed on dampened paper which shrinks when dried. Depending on the dampness then and dryness now, there is some sight variability in the measurements. Also, have you calibrated your ruler to be sure it is absolutely accurate? (Of course not, because none of us do. I don't.)
I have been in the field of metrology for a long time now. I measure things for a living. 1/4mm = 0.010 inch. Ten THOUSANDTHS of an inch. Experience has shown me that things that small are BARELY discernable to the human eye. It causes people to work extra hard at looking at the measurement - ie: with all that concentrating on one edge, then the other, sometimes people move the measuring device when going from concentrating on one side to concentrating on the other side. Also, even the thickness of a metal ruler (say 0.020" thick) will cause parallax issues with what is being measured beneath the ruler. And lastly, and perhaps most applicable here: what John Becker says. It is quite possible that even if you have the measuring task down pat, that the design may, in fact, be different from what it is stated to be. Paper dries out after the printing process and shrinks. It continues to change, depending on the environment it is stored in, and changes will occur over decades. The difference between what you see and what you expect is on the order of 1%. A 1% change in paper dimensions over decades is nothing out of the ordinary.
Your other metrics (perf 11x11, look on the gum side for slight ink offset, the 'raggedy' straight edge at bottom) are all only seen of flat plate printings. On the last metric, if you look at the edge under a glass, you will see that it is slightly 'ragged' - whereas if the bottom perfs were cut off with scissors or a razor blade, the edge would be much more 'crisp'.
Looking at your post of the back of the stamp: do you see the 'broken oval' of offset ink on the back? THAT is the offset that John Becker and I were talking about. VERY typical of flat plate printings.