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I would suggest using like flash units or checking the voltage polarity of the sync port if they are not.
Since flash units are designed to work on a variety of cameras, and cameras are designed to work with a variety of flash units, all will have the same polarity at the PC connection unless they've been tampered with.
The design was standardized years ago by the company whose initials are attached to the device: Prontor-Compur.
The sync voltage can vary between brands and models, but not the polarity.