Actually the two versions are printed on different paper types.
The 2531A (2531Ab full booklet pane) is printed on an essentially dead paper. When viewed under LW UV (black light) the paper appears a dark purple/purplish gray with sparse luminescent threads visible. The luminescent threads glow blue under the LW light and are easily visible under 10x magnification. It is also solid tagged. The paper appears white under normal light.
The 2531Ad (2531Ae booklet pane) is printed on a dull fluorescent paper. When viewed under LW the paper appears ivory with no luminescent threads visible. It is overall tagged. The paper appears ivory under normal light as well.
To see images of the tagging go to:http://www.stampsmarter.com/feature...ingView.html