I have a short bio of Stephens online that mentions his previous
association with Fiske and Gould as well as some others who may be featured:

http://www.twainquotes.com/UniformEds/UniformEdsCh26.html

Stephens also produced THE COMIC NATURAL HISTORY OF THE HUMAN RACE. The
book also presents a challenge to identify the heads. A scribbled note on
the end papers looks like others have attempted to identify characters in
this work:

https://archive.org/details/comicnaturalhist00steprich/page/n1

Barb