As far as i know, the "50" really has no meaning, BUT the DUAL does.
It was called the DUAL Sanitronic 50 because it was the first Kirby to have a 2 speed motor. Low for regular use, and High for hose suction.
If you notice the Speed Selector Switch, located underneath the Air Intake, and behind the Attaching Bar are 2 little push down type switches. One switch is for low, the other, high speed. All the regular floor attachments will depress the low speed switch,,the hose depresses the high speed switch when attached. The fan will not turn, when neither switch is pushed, and nothing is attached to the front of the Kirby, but the headlight will light. (if the bulb is working)
The Dual Sanitronic 50 is also the first time we are introduced to the Kirby Carpet Shampoo System, in those days called the Rug Renovator.
The DS50 is also the one and only Kirby to have a lighted KIRBY logo lense in the TOP of the Headlight Hood.
This is a sure way you can always tell a DS50 from every other 500 series type machine. (as none of the others have it)
My Grandma bought a DS50 in 1965,,it had a bronze tone to the "Sorocco" tan color that Kirby called it. I dont have the machine anymore but i did save the cord,,and it is a sparkly golden tan color that matched the machine.