Help with Old Mustang -

Maybe the OP is confusing rust with corrosion.

For at least the last 10 years all of the my distributors terminals have been made with some kind of white metal but more common problems has been carbon coming off the central post and grease from the points lube.

Doesn’t it use the typical black plastic distributor cap? The only place those could “rust through” seems like the 8 terminals that supply the spark to the spark plug wires. But don’t those have rubber boots over them to prevent water infiltration into the cap? Or did the rain water get past the boots? You can see the boots in the 4th photo in this link.