Andy,

You might want to poke around here as well:

http://www.railgoat.railfan.net/spcars/byclass/gon/index.htm

I don't have as complete photographic coverage of the steam era SP cars as I do their modern counterparts, but I think I have the the roster info pretty well addressed, with many thanks to Tony Thompson.

Is there a good reference source for the various SP GS classes?

I have some info on WP and D&RGW GS gons at the following which may be of some interest as well:

http://www.railgoat.railfan.net/other_cars/wp_cars/index.htm

http://www.railgoat.railfan.net/other_cars/drgw_cars/index.htm

I'm also curious as to what other roads had GS type drop bottoms besides SP

Lee A. Gautreaux - The RailGoat
http://www.railgoat.railfan.net/