I need the name of the control variable for SheetSpace/PaperSpace switching so that I can write a toggle command for this function.

Can anyone please help me?

OOPS!  I goofed.
That should have read: "SheetSpace/ModelSpace switching" -- as it applies when on a sheet tab for switching into and out of the modelspace viewport.
I'm currently using two toolbar buttons but would like to reduce that to a single toggle button.