This is a TPU all weather cap for the locking mechanism on Diamondback tonneau covers. Originally, my son wanted it to prevent the metal locking mechanism from marring his coated HD cover when folded all the way open, which is not a problem with my bare diamond-plate model but weather can be an issue.  This cap is designed to fit reasonably firmly. If you want to make it really secure for high speed driving, a small square of Velcro inside should do the trick.