The first thing that came to mind was a bit of toilet hardware... but that was just first impression.
The chain's not heavy enough for it to be a lock of any kind. Looks like it's just there to keep a couple of different parts from wandering off.
I think that the cylinder opens (unscrews or something) and the chain keeps it from going anywhere. There's probably an open space inside that holds whatever it's meant to. The ring is there so you can attach it to something else (like with a padlock).
There is indeed a protective casing that comes off and the chain holds it in place.
The ring you see is simply a handle, something to hold onto.
To tell you who would use it would be too much of a clue just yet, but it would not be a gunsmith and it isn't any type of reloading device.
It is an antique and not made anymore, though I'm sure there are other devices around now that fulfill the same purpose.
If you were ever around anyone that was using this..... you probably would not realize it.
I wouldn't say it was commercially used in the normal sense of the word, but it's not really a personal item. Most people would find it a novelty, but to some it would serve a very useful purpose even if only used once in a while. It would be useful while these people did their work or job or assignment....
You could be very close to this person doing their job and not even know it.
OK. I said I would post the whole thing after a few more guesses. Here it is, in all it's splendor. Complete with directional arrows. That probably have something to do with something...
Batter... err, typer up!