From what I vaguely remember as a small kid in the 1950s, those times are in the ballpark. I seem to recall leaving earlier than that from SF -- closer to 7 or 7:30 -- but your information is certainly more reliable than a 5-6 year old's memory.

BTW, the Plunge was still a Plunge at the time - slightly grody slimy and kind of dim (the fog never cleared that day) but fun. Later of course it became indoor mini golf and arcade...