Guffy,
The C and S stand for Conus and Spot Beam as you suspected.
Smithsonian will be on the C , the signals you have on the s are for your locals, maybe not
yours but those in the area, I'm not sure if you are getting your locals from D* or not.
The locals are normally on Spotbeam transponders .
Unfortunetly, the listing I have showing transponers and channels, does not have Smithsonian listed on it, I know, it's quite old now.
Someone here will be able to tell us what transponder it's on
Jimbo