Ivory Folding Rule

This is an antique folding rule from my grandfather. Unfortunately I'm not sure what is was used for or by whom (architects?). It seems to be made of Ivory and 'German Silver'. The metal part has pins to lock the 'legs'.
It has cm, inches and 1/4, 1/2, 1/16, 3/4, 3/8 etc. On the narrow side are unknown units I'm not familiar with: 10 = ca. 1.2inch or 3cm. The total length is 24 inch (~61cm).

This is what I found on the internet:
Chadburn Brothers Sheffield GB.
Active 1837-1898, optical/mathematical/philosophical & nautical instrument maker, Sheffield, England.

Here are some more photo's:
http://www.adrive.com/public/yxK6bg/Duimstok.zip
(total ca. 15MB)

Maybe someone can tell me more about this nice antique instrument?
Or point me in the right direction for more information?

All other info is also welcome: e.g. best way to clean it and make sure it stays in a good condition?