I have a few UMC headstamps but never seen a solid brass one. That is to cool. Turtlefoot is the one to talk to. He is with out a doubt the expert here. Cool find.
Congrats on a great find there! All brass shells can be hard to get a tight date on as in many cases the headstamp and other features never changed during the life of the shell. My eyes are not good enough to see some of the fine detail on yours, so I am going on a bit of a "hunch" by what I can see.
The head of you shell has a unique design to it, around the primer area. This was used on a few different all brass shells, with the most popular being the "First Quality". Although I don't have solid date information as to when this head style was used, it was featured in the UMC catalogs during the 1880's. My best guess is the 1880's, but it could be a little earlier or possibly a bit later.
I have a couple of brass 12 gauge shells a friend of mine gave me, he was in a mountain man club. I knocked the spent primer out of one and use it to play my slide guitar.