This round slider rule is marked Aristo 630 Chemie System Dr. Bückle and comes in it’s original leather Aristo signed sleeve. Made in Germany around 1960 for easy calculating of chemical substances. Fully functioning and in very good condition, only small signs of use or age. Diameter of the disk is a large ca. 22 cm or 8 7/8 inches. Check the below video on how to work your vintage man tool : circular slide rule operation instructions!