Had my cumuppance.
After declaring ordinary skeleton jacks are fine that on my Flea Jazz (2018 or thereabouts) started cracking loudly.
Turned out the (apparently) silver plate had tarnished badly around the contact area. Contact cleaner and a polish with a rough cloth restored function.
I'm assuming silver plate as the tarnish was black.
No they're not.
Acoustic instruments have them, and many semi-acoustics and some others (e.g. many Gibsons) have decorative ones and have rear-routed control cavities.
Yep not even tannin stains.
Originally three of the mugs featured glorified images of Trump, Musk and Tate. They disappeared, presumably upset someone who didn't realise it was satire...