Weinberg goes on to say that, where he came from, Memphis, the natural shoulder was unknown.

Twenty young men in what looks like a class photograph, every single one of them wearing a jacket with a natural shoulder.

Although Mailer himself is said by his biographer to have been pretty indifferent to the social side at Harvard, it sounds as though the WASP majority had plenty of cash and people running around after them.  Mailer, the Jewish outsider, had few clothes but wore them well (according to chaps interviewed for the book).  Richard Weinberg's family were puzzled as to why he had to wear some kind of uniform in order to fit in. 
I assume the shoes were sensible PTBs or something of that nature.

I wish that Richard Weinberg had gone into rather more detail.  I mean, the socks?  Somewhat later, it seems, following the publication and success of 'The Naked And The Dead', Mailer became something of a scruff: combat jacket and scuffed shoes.  I'm guessing, at that point, Harvard and its 'rulebook' hadn't rubbed off on him.  There's a photograph of him around this time wearing a plain white t-shirt: before that kind of look was popularized by Marlon Brando.  Otherwise the few pictures of Mailer I've seen over the years often show him in a denim shirt. 
But it was interesting that you had to wear the 'Ivy League'/'Preppie'/'Preppy' uniform at Harvard in order to 'fit in'.  It sounds as if both Mailer and Weinberg were happy to conform, at least at the beginning.

It's also interesting that, whatever else they wore whilst boozing or whoring, Mailer and his circle, knowing how to play the game, would climb into their establishment outfits if, say, they were going in to bat for Burroughs over the publication of 'The Naked Lunch'.