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It seems as if the use of PCB's in electrical insulation is associated with navy cable made during the second world war and and an early form of nuclear cable installed at Oak Ridge, TN during the same time period. That was sixty plus years ago and the information seems long gone.
Paper-oil underground (usually) power cable such as mass impregnated (Generally paper insulated lead sheathed cable or PILC.), high pressure pipe type cable and low pressure oil filled (LPOF) cable may have used PCBs as well long ago. These cable types may have also been contaminated by electrical utility oil filled transformers using PCBs.
The review WAI's list of paper cable manufacturers is as follows:
(2) Under "Product(s)", select "Power Cable-paper insulated" and:
(3) Press "Submit Query"
As far as the navy and the early nuclear cable go, you might ask the American "power cable-paper insulated" cable manufacturers to see if they can shed any more light on this subject because some of them historically manufactured navy cable and perhaps the early nuclear cable.
As far as other sources go, your best bet may be to keep searching the Internet using terms like "PCB contamination and wire and cable". OSHA and the Department of Defence may also be able to help you.
Sorry for the limited response but the information you are looking for was a long time ago and many large wire and cable companies have been sold, downsized or closed since then. Moreover I would imagine that many of the technical people who worked on these projects long ago have now passed on.