I was sorting through some Liberty series plate blocks recently looking for tagging varieties and I found an example of the 4-cent Lincoln on much brighter paper than all the others. In the series of pictures that follow the PB in question is on the right while I put an untagged example in the middle and a tagged variety on the left.
Under short wave UV, the normal tagged variety on the left lights up like it should but the one on the right looks a little off.
Switching to long wave UV, the brightness of the one on the right really shows up.
Looking at the backs with the long wave UV shows that it is the paper that is driving the 'glow'
My 2016 Scott specialized mentions the tagging variety but makes no mention of any paper varieties. Any US specialists out there (especially anyone fluent in Liberty Series stamps) have an explanation?
Hopefully the UV pictures aren't too dark to see.