As in the pictures, recent manchester united jerseys since nike took over as kit manufacturer have this "SECURITY LABEL" (black) inside or outside the jersey. on these tags there are 2 features which i doubt pirates would ever copy. the 2nd pic is from the 2008/2009 away jersey located at the bottom on the face of the jersey.
1. certain numbers and alphabets are made of a shinier silver thread which can be seen obviously from certain angle, different from the rest of the letterings.
2. there is also this interweaving stripe on the tags which on closer observation reads "NIKE SECURITY, followd by a nike tick"