I was born and raised 9 miles from the Elgin Watch Factory in Elgin IL.Lee is correct about the OVI being the Ohio Volunteer Infantry. The Mississippi Marine Brigade was made up of several states I believe Ohio,Indianna,Illinois and Missouri.You might see if the watch has seriel numbers that could be matched to a person. This is a start. http://www.elginhistory.org/
Sounds as if is sold already. You might tell if a connection is made to someone famous the value is worth more!!!!