I came across these wild looking derailleurs over on the bicycling Sub Reddit, and it just asked more questions than it answered. These are derailleurs mounted not below the dropout, but on the chainstay:
These models are from the 40's and 50's, before a standard emerged on derailleur workings. There are a couple differences in these designs - some are mounted on a tab on the frame, others are clamped on the chainstay; some work on a parallelogram, some work on a pivot, some on a plunger; some have 3 speeds, some have up to 6. A common feature seems to be that there are very few common features.
Clamped on the chainstay
Once of the shared features of most of these is that they were made by Suntour. the target audience for these bikes meant that the decision for this kind of derailleur was more likely to keep cost down, not for performance. While these systems worked fine, they had limitations with gears and maintenance. There were some advantages however; it was more protected from damage away from the rear of the bike.
The two below examples show how the same idea can be executed in such different ways, the only thing similar about these two is that they shift gears.
These styles were common surprisingly recently, with this example from the early 90's. This Schwinn could almost be mistaken for a normal derailleur:
This model took advantage of the parallelogram shifting to get a wider range of gears. It is not hard to see how this could have developed from or to a standard derailleur (the bike even has a standard derailleur hanger on it, for back up I guess).
More comments on the thread from equally interested and confused cyclists.