May be not iso resin, it was a blind guess as it was left as seen, not touched except by the roller, l have no clue what kind of resin they used in the seventies.
As there are the iso, ortho, vinylester and epoxy resins to my knowledge, l guessed iso was the most common in the early days. I might be mistaken thou.
The word in the title of the thread 'comment' not 'advise' is general, non committal and impersonal in nature.
However, it makes no difference who owns the boat.
Regardless of ownership, the treatment described is one possibility and we all learned from your comment a lot. Thanks! I agree with those, who would investigate.
The boat got antifouled, polished and launched.
It is owned by the same person since new and he is one of my permanent sailing mates. I on mine he on his shooting around on many days.
The rudder stock is rock solid, no play of any sort is detected.

Happy sailing again.