Control+shift+S is currently set to create a new surface.
If you really want to change this yourself and you like a quick -hack-, edit tcl/ac3dview.tcl (with Notepad or similar).
Look for:
Code:
# VERTEX MENU
acbind $top <Control-S> { ac3d createsurface }
replace "ac3d createsurface" with "menu_save_ac"
so you get:
Code:
# VERTEX MENU
acbind $top <Control-S> { menu_save_ac }
Note - all hacking disclaimers apply here