Thanks. Your boots-on-the-ground feedback gives me a feel for how it may have gone. I've got a 1964 Lineman & Cableman's Handbook on order from the library.
We've gone round and round on this on an airplane site, some arguing that the power company would have been careful to measure the breaks. My take on it was that the company would have done the measurements if asked, but probably not as a matter of course.
In a month or so I'll be publishing another version of my analysis. It will probably be my last one - don't seem to have any more courses of exploration. I'll put a note here when it's available.