I like to buy small bits of watch related items. I recently bought a Gruen case opener to go along with my Gruen pocket watches. Withe the opener came a device called a :Cartouche Watch hHolder"
What's it used for? Looks like some sort of holding device.

 

By the looks of things, a holder for the Cartouche line of tank style watches Gruen made in the 20s for women (and smaller men's wrists.)

 

Hi Geoff,

I suspect it's a movement holder, the screw is turned in to open the jaws and undone to clamp the movement under spring tension.

 

And Eugene's information makes more sense. It would be much good for circular movements, but perfect for rectangular ones.

 

Thanks guys, I hadn't thought about the tank watches

 

Early 20th century torture device. ;-)