These old starter motors and generators use brushes. To be more specific, ALL charging current goes through those brushes (unlike alternators). They do not last as long as modern alternators. The same holds true for starter motors, all current goes through the brushes. Rockauto.com sells brushes for $6-7. They are relatively easy to change. Ford provided inspection covers to show what's really going on.
It always puzzled me, why people let their starters and generators go until catastrophic failure when preventive maintenance is not only cheap but available. In The States, these original starters and generators aren't always available. When that happens, the prices soar.
If you're lucky enough to get an early warning, don't wait until failure catches you far from home. - Dave