Any differences in tennis balls over time are more difficult for me to discern vs. the changes in racquet technology.
(wikipedia on the tennis ball)
(ITF tennis ball history)
From the above, tennis balls look to be about the same weight now as they've always been (about 56-59 grams from 1965 to today), but they seem to use different types of balls in terms of bounce depending on the Surface nowadays. I'm not certain of what they did say in 1965 as to "bounce", though weight seems to have stayed about the same. Perhaps an expert on the history of tennis balls can shed more light on this issue. I'll have to look into it more closely to have a better informed opinion.