What is the purpose of those casing treatments and how were they designed?
The casing treatments shown work by providing an alternative path for the rotor tip leakage flows; which otherwise collect at the rotor tip when highly loaded and block the flow resulting in a surge.

The lack of change is because there was no significant improvement in computing power between the 50s and the 70s that would help with such designs, (in fact it is only very recently that the flows can be accurately modelled.) and the cost of the experiments required to substantially iterate the design would be prohibitive.

The difference in designs is partially driven by parallel development, partially by patent avoidance, and partially the desire to minimise the impact on the compressor's components and its efficiency.