Top Tube cable guides were not an all at once thing, but had gained momentum by the end of the 1970's.
Some just did not maintain their bikes, and rust formed under the clip.
Later rust formed at the edge of the guide. Probably faster as the small edges of the guide just did not provide a good place for paint to stick.
Powdercoat? Don't even. As the powder transitions to the semi luquid state it pulls away from this edges.