Just found this book put out by the old Federal Power Commission (now FERC).

babel.hathitrust.org/cgi/pt?id=inu.30000047597764;view=1up;seq=1

An excellent source of information - lists ALL the IOUs and munis as well as those coops that were in existence as of 1940.

For one, I finally verified that Central Gas & Electric Co. (aka "Cengas") and Central Electric & Telephone were one and the same.