Thanks, John! Greetings from Poland!
Can a bishop wear a watered silk mantle ?
It's not just a bishop but a cardinal (Gilroy, I believe), as revealed by the red sash and the red buttons.
Yes, it's card. Gilroy during sede vacante '63.
He is not a Cardinal. Rather a Nuncio or Apostolic Delegate. Both had the privilage of watered silk but in purple.
No, it's cardinal in mourning dress.
The picture is of Cardinal Sir Norman Gilroy then Cardinal Archbishop of Sydney, Australia.