Here we have a 6mm stump designed to fit popular jewelling tools. Stump is a 3mm post, but has a brass collet which can be glued/compressed on to the post to make it 4mm. I don't know which version of the SEITZ tool you have so have made a one size fits all part. The stump is designed to fit the recess in the SEKO 61/63 series plates to stop it moving while reaming.