Rim fire,
I assume you've read my infamous disclaimer . . .
so here goes on your Spencer Carbines . . .
#16041 . . . my SRS books show a Spencer Carbine, serial # 16040 issued to Company K, 11th Michigan Volunteer Cavalry on August 17, 1864.
#46819 . . . my SRS books show (closest serial # to yours) a Spencer Carbine, serial # 46854 issued to Company E, 11th Michigan Volunteer Cavalry in 1864 (no day or month given).
Hope this helps . . . if you haven't as yet, PLEASE read my Spencer Serial # disclaimer.
Two Flints