Scott lists two tagging varieties for this issue. 4138, Overall tagged and 4138, prephosphored paper. There is also a third variety which is a variation of the overall tagged variety 4138. The image below shows the two types of overall tagging.
One the left is the typical "green" overall tagging. On the right the tagging looks bluish. Taggant that looks bluish usually appears on stamps printed on hi-bright paper. The brighteners in the hi-bright paper are much brighter then the taggant swamping the green taggant color out and leading to this effect. If you were to examine a single stamp of this bluish variety your assumption would be that it was on a bright paper. In this case that assumption would be incorrect.
If you now examine these two varieties under LW UV you get a surprising result. Both are printed on a fairly dead paper. What is unique is that it appears some form of optical brightener has been added to the taggant. You can see this in the right stamp which glows a bright bluish white where the taggant is located when viewed under LW UV. You can also see they both have the same plate # V1111. Plate # V2222 is the prephosphored variety.
Note: the LW UV image has been manipulated to get a reasonable representation of the colors when viewed by human eyes. At least mine anyway. Pictures taken under LW UV lighting have a strong overall blue tint normally.