I think a lot of this has been answered already but I'll add a little more background.

The defrost button or lever is linked to the temperature sensing mechanism in the thermostat. The defrost mechanism will latch and stay in "defrost" position until the evaporator has completely defrosted and the thermostat sensing tube has warmed up to a point above freezing. While it is latched in, the contacts which turn on the compressor are blocked from closing; thereby keeping the unit stopped during the defrost cycle.

Back in the day - this was a useful feature. Envision this as your only fridge and all your fresh food was kept in it. You and your family work all day and don't have the opportunity to "babysit" the defrosting and turn the fridge back on once it's done. If you leave it off, your food would spoil while you were away at work.

So; with the automated defrost, one could actuate the defrost control and leave the fridge closed. The built up ice would start to melt, and in doing so it absorbs heat and would keep the food cold. Once the ice was all gone, the defrost control will disengage automatically, and the compressor will start up. So you come home from work and find the fridge running, defrosted, with water in the drip tray and no fresh food spoilage.

Frigidaire used variations of this. Even the later belt-drives had it. It seems that most had a separate lever which would latch until the evaporator warmed up, while the ones like yours have the push knob design.

In your first post, I missed your question about the letters and numbers on the dial.

The positions A, B, and C were for use in cooler weather or when the door wasn't being opened much. In other words a winter or vacation mode. It may not freeze ice in ice cube trays very well in these positions but it should not freeze fresh foods in the cabinet.

The numbered settings were for everyday use when the door got opened periodically during the day. The colder numbered areas were for heavy ice making or high use days such as when Sunday dinner was being prepared and many people were in the kitchen working and opening the fridge a lot.

The control is purely mechanical. There is nothing electronic in it. They are very intricate instruments and any changes in the parts, even less than 1/1000 inch (0.001") will have an effect on the operation of the control. Over the decades, the moving parts wear and the lubricant dries up and becomes tacky. As a result, the control doesn't operate at the same temperatures it did originally. From my experience, it seems that as they wear; they do one of two things... either they get colder for any given setting; or they start running with very erratic cycles and tend to go from not running at all; to running constantly in about 2 or 3 dial positions.

So it seems that your control might need some careful attention before long, to ensure it doesn't get worse.

Sincerely,

David