Rust or mold, it doesn’t really matter. As other have said it’s on the outer part of the circle - the bit contacting the outer glass thread. The inner circle is the plug that contacts the contents and is clear.

If it feels scratch with a finger nail its rust, if it’s soft and scrapes off its mold. But as I said it’s not in contact with the contents so it doesn’t matter.

Also the contents of the jar are pickled. That means brine or vinegar, which is highly acidic and is what keeps the food fresh/prevents mould and bacteria. So if the pickles themselves look fine then they’ll be fine to eat. If the pickling had failed the contents would be mouldy.

Rust would make sense as the content of the jar is acidic and acids accelerate rust. There could be small pockets of air left at that location when you seal the jar and some fluid inevitably gets forced out as it is sealed; air plus acid is perfect for rust. But the jars internally themselves were otherwise well sealed as there is no rust in the inner bit of the circle, suggesting it plugged the jar contacting the fluid directly and no gas was left.

This likely reflects the jar lids are not quite perfect for the jar or possibly not screwed on to their perfect max tightness leaving air behind at those locations. But they were screwed on well enough to seal the content.

AI mangled or just compressed multiple times as its been shared around? Probably a download of a compressed version a few times before ending up with this text? It looks lower resolution and cropped rather than substantially changed to me.