You are not gauging the perforations correctly. Note in your image how the perforations are not exactly aligning with the gauge.
If you try 10 1/2 (on the short sides) you will see that the holes line up much better.
Of course there is another way to ID this block since it has a plate number. Use the Plate Number Lookup tool on Stamp Smarter site and it will return the identification. Here is the linkhttp://www.stampsmarter.com/feature...ockHome.html
Click on "Search By Plate Number" button then enter plate number 21379 and it will tell you what stamp it is.
Here is more informationhttp://stampsmarter.com/1847usa/192...ntifier.html