This interests me a lot actually! When you stop and think about it, pre-bikini era, it would be highly unlikely for you to ever see anyone else's belly button. Unless you were married, of course, but even then, you would only get to see your significant other's. Were people not aware of different shapes/innies vs outies back then? Would they have compared them? Or would all of this been too "taboo", even though it's something most people do quite innocently among friends.
We're spoiled in these times that we're able to see the bellies of most people we know at the click of a button. Pre-war, you would have to marry the broad to see it!
But I know you're looking for pictures and not Witchery's rambling... ::)
These are 1950s to 1960s I believe:
And here's one of my favourites... the lovely Caroline Munro:
