Not sure why the San Bar cards are doing that. Don't recall that ever being an issue. Have you tried reversing polarity on the line INSIDE the set?
I seem to remember that there was a provision on ITT sets to disable the polarity guard by cutting a strap on the back of the Tone Dial. I don't know if that would apply to WECO sets, I'll have to look.
Nice job on the shoebox!
Sam
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