There was a misprint in the printed instructions included in your box. Your Werkstatt will assemble and work with or without cutting a section from the black washer.
Your Werkstatt should not have a metal tab on the output jack.
Some early versions of the printed instructions referenced cutting a section of the washer to make room for a metal tab on the output jack. That metal tab existed in earlier versions of werkstatt, but is not present in the current version.
The Werkstatt Assembly Instructions pdf on moogmusic.com reflects how to build Werkstatt without cutting the washer.
The print instructions in the box have also been updated – only early serial numbers are packed with instructions that talk about cutting the washer or locating a metal tab on the output jack.