Also, at that time, the largest "standard ampere rating for circuit breakers" was 800 Amps. Fuses had standard ratings up to 6000A and there was no restriction in the '59 code for going to the next higher standard rating...
Then, in the '62 code, there was a change that required an 800A limitation on going to the next higher standard rating, and they combined the wording so that both breakers and fuses were dealt with in the same exception.
Considering what Creighton has said about there not being any comment about the change in the '62 edition, I infer that the code change was done to clean up the disparity in language between the two types of overcurrent devices. Since it doesn't seem that the device technology (breakers vs fuses) was at issue, then I am thinking that it must have been a practical issue of putting a limit on the amount of space taken up by parallel wires. They might have noticed that switchboards weren't allowing enough wire bending space for a larger parallel feeder. In the '59 code, the ampacity of 500MCM THWN wire was 380 amps. Putting them "in multiple" (as they used to refer to parallel conductors then) would provide an allowable "current carrying capacity" (ampacity) of 760A..which would be just fine for that standard 800A overcurrent device.
