Strongbow,
The SRS book Volume 4 indicates that a good number of Spencer Rifles issued to the 72nd Indiana Volunteer Cavalry were issued in April or May of 1863.
The lowest serial # issued to the 72nd is #1958 issued in April, 1863. After #1958 there are many different units that received Spencer Rifles, and NOT UNTIL serial #4502 do you see a very long listing of Spencer Rifles, one after another, issued to the 72nd, all the way up to #6308, issue dates 1863 and 1864, and then other units received the Spencer Rifle along with the 72nd . . . again all the way up to #8807 and beyond . . . very scattered issues to many different units.
Perhaps the 1864 issues were indeed replacements.
Two Flints