Okay, so here's what I know.
Here are photos of some Red Crown thermometers in situ.
In the first one, please note the Ford sign. This sign was released to Ford dealers in April 1917 (per that month's issue of
The Ford Times) Later versions included "Fordson" (post-WW I tractor production) and one that includes "Lincoln" (purchased by Ford in 1922).
The second photo shows a Glascock Coca Cola cooler. Glascock obtained the rights to manufacture these coolers in 1928 or 1929.
The third shows an obviously 1920s car at the pump.
Now, as with everything else it is important to remember that once a sign, thermometer, gas pump or tire rack was put up, it did not come down because a later version was available. Also, a photo showing a car could be made any time after the car was made. So a 1920s car can easily be in a photo taken in 1933 or 1935 or during WW II or last week for that matter if skillfully posed.
The rule of thumb here is: "No older than." If a photo shows a 1922 Ford Model T, the photo can be "no older than" 1922. As with our Glascock cooler, that photo is "no older than" 1929. But, the thermometer could have been tacked up in 1920 for all we know.