If you wear a size L, I think a true 54 is going to be too big for you. It should be right around 27", side to side (across the number), laid flat. It will be longer than the replica L, too.
I don't think they made any 'authentics' with the SB patch. Shoot me a pic. These days, I assume every jersey is a bad knockoff, unless I can see it.
You can't wear a fake jersey with a legit watch