That is where the knobs stop in the 1x range. You should see that there are two small windows above the knob and behind the numbered collar. The "5" on the clear collar should stop above the left window (illuminated when scope is "on") situated behind the clear collar. The 10/20/50V ranges are attained by using a properly equipped TEKTRONIX 10X probe which will cause the left side window to go dark and the right side window behind the collar to then illuminate.
The Window to the left is illuminated when the scope is turned on (if the circuit and lamp are not damaged) and the Window to the right should come on when a TEKTRONIX 10X probe is used which connects the contact ring of the BNC to the ground through a spring loaded pin. This is called "probe coding". I hope My attempt to explain makes sense.
--
Michael Lynch
Dardanelle, AR
Michael W. Lynch