IC's with dimples on covers

The photo above has packages with 4 dimples on the cover.

The package by itself at the bottom is the same, and even shares a near datecode with the above, but has no dimples.

Anyone know why the dimples?  These chips don't get hot and would not have thermal cap in operation.

And for those who want to drool, the 3 ICs with windows are Intel 1702 pulls he has.

Here are closeups of just a single package from each photo.