Freon came out in the 1920's, but GE's Monitor Top fridges used SO2 for years. It looks to me like late 1930's and perhaps into 1941 GE fridges moved the cylindrical compressor assembly from the top of the fridge to a space under it, resulting in a raised refrigerated compartment. I presume these still used SO2.
I have a 1948 GE one-door fridge. It still runs well but needs a new door gasket (I have the material, but haven't gotten round to installing it... I've been using the need to repaint the exterior as an excuse). According to the tag on the compressor assembly, it uses Freon-12.
The thinking with these old SO2 fridges apparently was that the odor of leaking SO2 was so noticeable (think rotten eggs) that people would clear the area before it became a serious health problem. However I've also read that the SO2 could react with water to become sulfuric acid which could cause skin burns and breathing problems. On the other hand in the older leaky homes of the era, the quantity contained in a residential fridge was not though to be lethal.
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