Not quite, although that link will definitely prove useful in the future!

For this I'm looking for straight counterbores to put an allen-head (socket-head) bolt into. So, straight cut down, not tapered.

For example, (numbers are not correct but for ease of explanation):

- The adapter thickness is 0.750"

- The through holes need a diameter of 0.500" 

- The counterbores need diameter of 0.750"

- The counterbore only needs to be 0.500" deep (2/3rds the way through).