One of the best sources of information for identifying and dating electrical
apparatus and line furniture, especially fuses and insulators, are trade
catalogues from manufacturing companies such as General Electric, Westinghouse
and Amalgamated, etc. Those are relatively easy to find at public libraries,
archives and technology museums. Also, several good guides on glass and
porcelain insulators specifically have been published. I can send bib off-list
if you are interested.

Gerard Gusset
Ontario Service Centre
Parks Canada