Am sure Two Flints is aware that if you Google:
"Spencer Rifle Manual of Arms", you will see a National Park Service version that includes a comment that no official manuals were adopted. NPS adds that there is none for the Henry, either, but theirs is adapted from an unofficial 1864 (Spencer) "manual" written by an Ohio officer.
Download and print it. It is as close as you can get.
There is also a fine article in the September-October, 1996 SKIRMISH LINE Magazine, by A. M. ("Tony") Beck:
"Spencer's Repeaters -- Some History and Shooting Tips". I can send you a copy. Kevintinny@hotmail.com.
By the way, when searching for Spencer articles, one appeared from Civilwarprofiles.com "Col. Wilder Spencer Rifles".
In THAT info is a comment, with no reference, that Col Wilder first wanted Henry rifles, but could not get them, so he went with available Spencers! I can hear teeth gritting. Haha, with respect.
All the best,
El Supremo/Kevin Tinny