Mystery Object: What is it?

It’s time for another Mystery Object! In past rounds I’ve posted pictures of things like glove stretchers and vinaigrettes for you to identify. This time, we’re going to try something a little different…because I don’t have any idea what this Mystery Object is! Do you?

Here it is:

From the end of the jump ring (which makes me wonder if it’s something that might have been carried on a chatelaine) it measures 2 ¼ inches. It’s marked “A.M. Co” and “STERLING SILVER.” The main length of the body is machined in tiny ridges, and there’s a slot running its length. When you turn the knob on the end, a sort of tongue protrudes through the slot:
The object easily comes apart into two pieces--an inner core and outer shell--and has a little catch so that I think it’s made to pull apart. The inner core with the tongue pretty much fills the outer--there's not a lot of space in there:
Unfortunately I don’t have a date for this item: an on-line search for “A.M. Co” didn’t turn anything up about the manufacturer, and neither did searching on silver hallmark identification sites. I’m not even sure if it’s American or British.

I have a couple of theories…but I want to hear what you think. No theory too crazy! And anyone who can provide a positive identification for this object will win a signed copy of your choice of Bewitching Season or Betraying Season.

Let the guessing begin!