Originally Posted by
roughkype
Just looked at your pictures again. The rust spots are in the same place, more or less, on opposite sides of the tang. That suggests the scales might be a little fritzy. Worst-case scenario: celluloid scales exhibiting cell rot. They break down and release either acetic or nitric acid vapor, depending on which kind of celluloid it is. To tell if they are celluloid scales, heat the end of a paper clip in a match or lighter flame, then touch it to some part of the scale you can't see. If you get a little waft of piney-smelling smoke, you've got celluloid scales. The only thing you can do when the scales start going bad is get them off the blade before they damage it any further.
The other thing that could be happening is if they're horn scales, some sort of moth or beetle larva can make its brief living and munch out a little cavern. The moisture from their life, and also held by the little cavern when they've moved on, can rust the blade.
It may well be neither of these things, could have been the blade was put away wet many decades ago. But do check the celluloid thing; that can get pretty bad.