View of a small coax with its shield reported to a blue wire, itself soldered to the same trace as the positive supply on the red wire. https://assets.chaos.social/media_attachments/files/113/800/582/289/883/106/original/0a96868512646986.jpg
@altomare starting analysis of the second board. As suspected, the coax shield is connected to the positive supply, so this oscillator is actually powered by -24VDC to ground... Or, output signal is referenced to +24V. Weird.