@1typesetter. I was told a while ago, that it was air bubbles in the ink that caused this type of inking error. And it always seemed to be a reasonable explanation, especially because the majority that I saw were only small circles, but apparently I was misinformed.
I want to thank you for the clarification, and the links, they were very informative. I always appreciate learning something new.
The stamp that I have pictured above came from a sheet that I purchased about a year ago. The sheet also had quite a few other stamps with the same type of error.
I decided to check some older pics of EFO's I have of this issue on my computer. And to my surprise I found a pair that I had from more than three years ago that seems to match part of a block from the current sheet.
Check the two pictures shown below, one of a vertical pair orig. scanned on 9/16 and the left pair of a block orig. scanned on 2/19. Many of the inking flaws seem to match up perfectly. I cropped both pictures to try to make it a little easier to compare them, but don't know how to get them side by side.
Here are at least 4 very similar features
1) The white void at the upper left, especially the circle in the extreme upper left.
2) 2 or 3 of the circles in the top middle
3) The 2 circles to the right of the "6c" including the broken ring.
4) 1 of the rings to the right of the sheep's head

