It is a Flat Plate #610.
The rare #613 rotary sheet waste stamps is among the most misidentified U.S. stamps.
As can be seen in the above illustration, the Harding rotary stamps do not have the sharp clean look of the flat plate printings.
Rotary stamps can be identified by the size of the stamp. Rotary coil stamps are wider and rotary sheet stamps are taller than their flat plate counterparts. Although the rotary coil waste stamps are somewhat scarce, it is the rotary sheet stamps that are really rare. The exceptionally rare perf 11 rotary stamps can be distinguished by the fact that the design measures at least 22½ mm tall, whereas the common flat plate stamp rarely measures more than 22 mm tall.
It is possible the reason for misidentification of this stamp is the fact that the measurements noted above are very tiny, a fraction of a millimeter or two here or there and a common stamp becomes a rare one. Compound this with the fact that no two people use a ruler identically, and you have a recipe for disaster. That is why the template method can not be overemphasized.
You can learn more about the template method of measuring, by either searching this forum or using the Stamp Smarter website (click on link below).