An article in the Geldscheine Online newsletter pictures an interesting mechanical interest rate calcluator medal issue by a German bank in 1927. Here's a Google-translated excerpt of the description. -Editor

The interest calculator, which was certainly given to customers as a promotional gift, comes from the happier days of the Danat Bank. It consists of two embossed aluminum discs with a diameter of 60 mm, riveted together in the middle and can be turned. The edges are slightly notched to improve handling.

The obverse shows the imperial eagle in a pearl circle, above it in a semicircle in two lines "DARMSTAEDTER UND NATIONALBANK / KOMMANDITGESELLSCHAFT auf AKTIEN" and at the bottom "EigenkAPITAL 120 MILLIONEN RM". The marginal writing on the back reads "THE WAY TO PROSPERITY" at the top and "DAS BANKSPARBUCH DER DANATBANK" at the bottom.

In the middle then the seven-line text of the interest calculator: "RM 10,000- / SAVE / IN O YEARS AND O MONTHS / IF YOU PER WEEK / O RM AT O % / ON COMPOUND INTEREST / INVEST", including the company logo. The individual values ??can then be read in the recesses. To do this, only one of the discs needs to be turned.

There must be similar pieces from around the world, but I'd never seen one quite like this. -Editor