An extremely rare brass sliding rule for mother of pearl buttons by Sally Polak Amsterdam.

A brass sliding rule. Overall length 7 cm. Fully extended 13,5 cm. The middle part of darker coloured brass. On the measuring side with 3 different scales marked "L","M", and "P". On the back: "SALLY POLAK LTD AMSTERDAM PEARL-BUTTONS". The rule was used for measuring mother-of-pearl buttons. The firm was active in the first quarter of the previous century at the Zwanenburgwal 35 in Amsterdam. Only 2 slide rules remain: one in the Jewish Historic Museum In Amsterdam,  and this one which used to be in a private collection. It is in perfect condition.