PCLister, welcome to the forum. First of all, you ought to go to
http://stampsmarter.com to find some good info on how to ID your stamp.
The dimensions of your stamp really do not mean much, and you should use the perforation gauge correctly. The black dots on the gauge should fit neatly and perfectly in the perforations of the stamp. Besides all of that, I really do not think you have a 596
Peter