Rimfire,
Not sure I can help with your 51420 - nothing really definite. My SRS book shows:
serial # 51417 - Company A, 7th Indiana Volunteer Cavalry, issued 3-1-1865 and
serial # 51423 - Company I, 19th New York Volunteer Cavalry, issued 10-1864
Most 514XX serial #s were issued to the Indiana unit. Hope this helps.
Two Flints