Ah. The set_prop_str was for disabling the Canon display:
http://chdk.wikia.com/wiki/PropertyCase#Digic_III_.26_Digic_IV_.28propset2.29
I understood that you were trying to use it to set the display mode. However, for some cameras that is the wrong index for that value ( propset 1 ). And according to the wiki documentation, it is "read only". Which mean that setting that value will not set the display mode.
So I supplied an alternate way to set the display mode.
