OK, think I found it ...Dialogs\\Plot Stamp ..."PlotStamp" but I see 0x00000000 (0) for off and 0x00000001 (1) for on. Do I need to pass the entire string to change it or just a 0 or 1?
I'm passing the entire string and I see the value is changing in the registry just like it does when I manually place the check mark in the plot dialog BUT if I manually turn off plot stamping, run my lisp, the registry value changes but no plot stamp. I'm guessing there's another registry setting to change somewhere.... :-(
My guess would be that the registry is only read when AutoCAD is opened... If you change the registry entry while AutoCAD is open, AutoCAD will still retain the value that it got when it read the registry when AutoCAD opened. My bet is that AutoCAD will even write out it's current settings when it closes, so changing a registry value while AutoCAD is opened would not be the way to do it....
There should be a Visual Lisp route to change the Plot Stamp setting (I'm no expert at VL... I"m an old school LISPer...) but it should be there...