This round slider rule is marked ALRO Syst 200R, comes in it’s complete metal case, with the original box (loose on one side and missing some of the paper wrapping) and the original manual in Dutch. Made in Holland in The Hague somewhere in the 1950’s. It has the patent (pending) numbers in the centre. It has a light aluminum case with brass studs in the middle and plastic centre cursor. It is in good working condition with hardly any signs of use and age. Closed it measures ca 13,5 cm or 5 3/8 inches wide. See pictures for details.
Check the video on how to work your vintage man tool : slide rule operation instructions!