Looks like a later die state. Note how the devices towards the rim are making a break for the border? That is what is going on. On a doubled die the devices will be enlarged in the center of the devices: 1968-D
Sorry 1961 D! Yes it is very faint and I didn't see any corners splitting. I had a 1964 with the 4 doubling up and you called that one a Die Deterioration. Thanks Coop! So doubling usually occurs left to right not up and down?
More in the center of the devices. On doubled dies it can be stronger from right to left of vise versa. Or the same all across. It can vary. I was reading today about the die classes on Mike Diamond's site. http://www.error-ref.com/doubled-dies/ Explanation of the different die classes. A very good read.