Tektronix's 465B Analog Oscilloscope Volts/DIV's Question
I imagine the 10, 20 and 50V markings are for when the probe is set to 10x. The dial should have two bands to correspond to 1x and 10x settings.  

I have an old tek scope which does this - the highest you can turn to for 1x is 5V/div.  

I just googled for a picture of the 465b and it looks like this is the case - notice there are two bands on each knob. On the bottom knob you can see one is at 1V/div and one is at 10V/div, which would correspond with 1x or 10x setting:



EDIT - I have added arrows pointing to the bands to clarify things.