I'm no expert, but I'm pretty sure this post-dates '65. And my guess is that it's got at least a 100 amp main, if not a 200. I'm not an electrician, nor an expert, but I've done a fair bit of house wiring and I recognize older hardware with the "Square D" brand as being generally considered high quality. In recent years, Square D has watered down their brand name by adding a "home line" line of products that are more affordable, but made of flimsier materials. The fact that it was high quality hardware and it's showing this much corrosion is part of my concern.
A lot of "me too" companies made breakers that would fit a Square D panel. If Mal is lucky, his breakers are Square D brand (suggesting higher quality than some of the knock off brands) but even so, I don't think they are considered to be "sealed" and immune to corrosion, and I don't think you can safely assess their condition by inspecting them unless you take a saw and cut one open. The breakers themselves aren't usually very expensive and one thing to consider is just swapping out the breakers and leaving the rest of it as-is if you're on a budget.
One thing I learned about breakers is that at higher voltages and amperages, they aren't necessarily as reliable as they are for 15/20 amp 120 V circuits. My parents had a breaker for an electric dryer that failed by melting a hole in the side of the breaker and melting the contacts, rather than tripping, when there was a dead short circuit inside the dryer. (It was an Arrow-Hart Murray breaker, 240 V, 20 A circuit). The electrician who replaced it for my parents mentioned that he had seen more than one example of that mode of failure on 240 V breakers. That was a lot of years ago and maybe things have gotten better since then, but there was no corrosion in that panel and it was only a 10 or 15 year old installation when the breaker failed that way. By the way: that failure mode is one reason why you should always keep the door closed on your circuit breaker panel when you don't have a reason to have it open.
2018 Niro PHEV Gravity Blue