I’ve noticed the corroded areas where the scales touch the razor too Oz. After seeing so many with ho other evident ‘water wear’ I wondered if the scales themselves could do it. Research did not yield much, but this material preservation site had an interesting mention for cellulose nitrate, it can react with iron. Cellulose acetate didn’t have the same caution though. Scroll down to hazards and safety.

Cellulose nitrate - CAMEO