Here are examples of 2720 (top stamp) and 2720a (bottom stamp). I don't have 2720b but the pictures should help with identification of any of the three. I highlighted what to look on these. These are taken under SW UV.
The red boxes are areas you can look at to determine if the block tagging is present. Since the block tagging isn't typically aligned perfectly as stated above on the left side in these case you will see a darker looking strip of color. On the right side in these cases, you see a brighter strip of tagging, greenish yellow in the picture.
The yellow circles show the easiest place to look to determine in 2720 or 2720a. In the top are the dark feathers which indicate printing is on top. In the bottom, the feathers are muted. Hope these two pictures help.