Little assistants to help align stuff on the PROXXON MF70 micro mill (and presumably the standalone X-Y table by the same manufacturer). The slot is 6mm and the hole 6mm. UPDATE 29.3.2015: I made one out of aluminium. I used a 3mm end mill so the inside corner has a radius. The stock I had was 5 mm thick, so that determined the height of the piece. I have used the plastic ones successfully. They are good for aligning your stock on the xy table of your mill so that you can get the most out of your stock.