How did they keep the tires balanced? On modern rims they would balance the spare as well as the ones on the ground. Or didn't they go fast enough to cause the tires to shake & shimmy?

It is an inflated spare on a metal rim, you can see the valve stem at 12 o'clock. In the event of a flat the wheel and spoke assembly remain in place. The rim and tire come off by removing the six clips that secure it to the wheel. A few months ago in Shorpy there was a picture of a guy replacing a tube on the side of the road in the depression. But he was a farmer used to making things work every day of his life. I'm thinking this attractive lady could change out an inflated rim with a jack, tire iron and a spare set of gloves.

If you blow up the photo a bit you'll notice the spare is mounted on a spoke-less rim. You can even see the air valve at the 12 o'clock position. To change the tire all one had to do was jack up the car, remove the six nuts that hold the tire on, remove the flat and replace it with the spare and replace and tighten the six bolts back up. Easy!

The side-mount spare appears to be just a tire casing carried on a Y-shaped rack. It does not appear to be a spare wheel. (Please educate me if I'm wrong; I don't remember ever seeing a Y-shaped tire carrier.)

Mounting a tire is something which few of today's motorists would, or even could, attempt at the side of the road. Getting a modern tubeless tire off and onto a modern rim without a tire machine would be quite a challenge, if it is even possible.

Fortunately, we have such better tires that tire trouble is now fairly rare.

Is that Ruth Buzzi and Arte Johnson on the bench?