Many creative and interesting guesses were made. Several guesses came close.
The most popular suggestion was a calibration tool used to set explosives during World War I.
Shrapnel shells were widely used in WWI as anti-personnel weapons, and were particularly effective in the open. “Shrapnel” is a term often misused to indicate fragments of bursting high-explosive shells, but true shrapnel was a shell that burst over or in front of the target, throwing a payload of small balls forward and downward. The shell was detonated by a fuze, which in WWI field artillery was frequently an igniferous time fuze. Time fuzes allowed the shell to burst in the air at a certain time after being fired.
Until the development of electronics such fuzes were set manually with a key or wrench. Our mystery object is a German-made example.
Igniferous time fuzes consisted of a powder ring in an inverted ‘U’ metal channel. The shock of firing the shell ignited the powder ring, which would be transferred through a flash hole to the magazine and ignite the bursting charge in the shell. Setting the fuze aligned the flash hole with a time marker on the channel. These fuzes relied on their velocity to be effective, and obviously setting the fuzes manually required great skill – particularly with moving targets, as frequently happened.
Shrapnel shells were gradually superseded as an anti-personnel munition from World War II in favour of high-explosive shells, filled with explosives in a casing which would also fragment.