The computer is placed on the anti-static pad and is grounded. It is therefore not necessary to attach the black clip to the computer case.
Open the computer and remove the lit. Put the lid on the edge of the anti-static pad. This will give you additional grounded area for putting things aside while you work.
Upon opening this computer I noticed that the lid had come from a Mac Pro 3,1. This is an earlier model. The lids of the 3,1 are interchangeable with the lids from the 4,1 & 5,1. Here is evidence of careless work practices of at least one prior owner. Upon inquiry I was told that the item was exchanged to give me a cleaner unscratched lid.