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I’m going to take a really wild guess at this, so please treat it as such …
The front seal of the compressor failed or the compressor was rebuilt. Either of these require the removal of the front pulley/nut. The pulley was messy, so the person who serviced it cleaned it with an excessively strong solvent, removing paint and oils. This allowed rust to easily form even with light moisture.
That’s my guess, take it for what it is …