You have probably long-since fixed this, and I am just now visiting this site after a long hiatus, but I have a 1974 10M and have seen this. I'd start by just taking a triangular scraper--an old-fashioned can opener works great!--and scrape out the crack and see what you find. I suspect it's more nuisance & cosmetic (maybe hitting something & a small crack) than serious (the whole rudder tube destroyed). Hope it was the former.
