Talk about being a survivor! Impressive.
The only thing is that there will be few patients from that time to share experiences with, and probably even fewer oncologists.
So my comments are speculation. But I think it is likely that even 50 years ago they will have taken efforts to ensure the radiation doesn't spread beyond their target - with testes that should have been easily achievable with lead shields even if the machines of the time weren't as tightly focussed as now. If that was the case, you probably remember, there is no obvious reason that you would be more in risk of incontinence than any other patient undergoing radiotherapy. In general with modern machines they can target the beam to minimise side effects, not that anything is guaranteed.
Good luck. As far as I know incontinence is not often a big issue with radiotherapy these days, and the odd bit of leakage easily managed by wearing a pad. But that will also depend on your particular situation and treatment, so you need to discuss with your oncologist.