The Bell System History book 1925-1975 explains all this in detail.
> [TELECOM Digest Editor's Note: Lisa, where would one obtain this
> 'Bell System History book' to which you refer? PAT]
The proper title is:
" A History of engineering and science in the Bell System vol. 3. Switching technology (1925-1975)", published by Bell Laboratories.
This should be found in most larger libraries, such as college library. I strongly recommend it.
It is part of a series of books on the history of the Bell System. I also recommend volume 1, "The early years (1875-1925)".