Hi,
Perhaps Al or Mark will reply, they know more than I do about tagging. But to my eye, this appears to be the tagging mat showing wear. You can see a very similar wear pattern on the example in the Stamp Smarter Tagging database here
http://stampsmarter.com/features/TaggingView.htmlClick on 'View By Catalog Number' button then navigate to #1895a, scroll down to the Narrow Block and see example of mat wear (click on image to see full size).
Don