FinnO,
Please a better, CLOSE UP photo of your brass gizmo on the front sight and explain how it is supposed to work for you . . .
Your Spencer Carbine, serial #39259 - same # should appear under the forearm on the underside of the barrel - serial #s should be the same as on top of the receiver . . .
Your Spencer may have been issued to Company L, 2nd New York Volunteer Cavalry in March, 1865.
Two Flints