Rob63,
Did a check on your serial # 33390. My SRS books show serial #s 33305 & 33448 as Spencer Carbines issued to Company E, 19th New York Volunteer Cavalry in September, 1864. Perhaps (did you read my disclaimer) your Spencer Carbine went to the same unit as 33390 falls in between the above serial #s??
Hope this helps!
Two Flints