That is such a finely made hat, I love the soft bow and beautiful color. I am not seeing a Bonwit Teller label, I see a Mr. Arnold label inside the hat. Perhaps there is also a Bonwit label?
I agree it looks 1950s, maybe the later part of the decade and could even be very early 1960s. That style continued to be popular throughout the 1960s, actually. But I think (not sure) that "Mr. Arnold" stopped designing by the mid 60s. That is not his early label, hence my dating it to the later time period. His early linings were hand painted with his name and the little mirrors were gold and actually had silver gilt for the glass and sometimes glued on plastic jewels.
It really is a beautiful hat.