It is the double-exposure switch at the right side of the view lens ("1").
When it is pulled down and the red-colored dot is out, double-exposure is ready and when it is pulled up, double-exposure prevention is in function.
At the left side of the view lens is the M, V, X switching lever ("2").
(Only "X" is left on Rolleicord Vb-type 2 since 1968.)
M : Flashgun, X : Electronic flash, V : Self-timer
In the right picuter above shows the flash sync contact.