Curtis,
I assume it's a carbine . . . your serial # is 18119. And I assume you have read my Spencer Serial # Disclaimer! Your #18119 MAY HAVE BEEN issued to the Michigan unit mentioned below.
# 18125 was issued to Company K, 11th Michigan Volunteer Cavalry on August 17, 1864.
Unfortunately, # 18114 was issued to two different units?
Apparently it was first issued to Company M, 12th Illinois Volunteer Cavalry on March 1, 1864, and perhaps, re-issued to a member of Company G, 18th New York Volunteer Cavalry on May 10, 1865.
I thought by giving info on #18114 & #18125, it would shed some light on which unit received your # 18119. My assumption is just a "guestimate". Read my Disclaimer! #18114 & #18125 are the closest serial #s to your #18119.
Hope all of this is of some help.
Two Flints