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This is a common problem for you Bikeinfantry - but not for anyone else it seems! I had honestly never seen this before you had 2 problems with the same handle bar. The bike is from 1965 and I have older bikes (eg. 1952) where the handlebar is rock solid. As you have suggested it could be an issue with the metal on this particular model - because of the high aluminium content it is susceptible to metal fatigue after a few decades. Also I don't think your problem is caused by rusting from the inside - the layer of oxidised metal seems pretty small.