The picture below shows the difference between 3843a the block tagged version versus 3843d with the block tagging missing under SW UV light. I find having color pictures of what to look for is helpful. If these are helpful to others I can post images for others stamps as time permits. 3843a is on top and 3843d is on the bottom. I only show two of the stamps for clarity.
3843a - Highlighted by the yellow box is where the block tagging is located on each of the individual stamps. The image also looks like it has a haze over. If you only have one stamp, another area to look at is the edges around the image. In the image below I have highlighted two of the edges where the block tagging doesn't align exactly with the main image. It shows up as a darker band since this area isn't covered by taggant.
3843d - In the bottom stamp the image will much appear sharper and without a haze.