The nubs you are looking for are on the perimeter of the pad and face down. They are located in about four different locations around the perimeter so easy to see. If you put your hand in and feel the bottom side you can feel the nubs (nipples). There are usually 1,2,3 or 4 indicating the pad thickness as follows for most MBs:
1= 8mm.
2= 13mm.
3= 18mm.
4= 23mm.

Tobias MB
4 MBs