Union Metallic Co. (1867-1911, Merged with Remington to form REM-UMC
from 1911-1934, Bought out by Dupont to form Remington Arms Company
in 1934 and is the same as of present.
The shell you have is somewhere between 91-135 years old. I would not
try to clean it other than an olive oil bath for a month or two. As I
am sure you are finding out, they can be very fragile and due to the
age and thin nature of the brass it will fall apart if you put it in
acid washes or use a brass cleaner. In the best of cases the acid
will eat right through some of the metal and leave pitting. Ugly! I
am listing a site you can go to to find more on the shells you are
going to find. They are affectionately referred to as the 12th GA
(Georgia) buttons on the Civil War forums.