custom-variable-reset-saved

custom-variable-reset-saved is a compiled Lisp function in `cus-edit.el'.

(custom-variable-reset-saved WIDGET)

Restore the value of the variable being edited by WIDGET.
If there is a saved value, restore it; otherwise reset to the
uncustomized (themed or standard) value.

Update the widget to show that value. The value that was current
before this operation becomes the backup value.