The big difference is the outer material. It feels more natural and softer if that makes sense. A real PL has a certain nylon smell that you learn to love, especially after it is washed a few times and broken in.
For instance, here is a real label for one made by Royal Maid.
The liners date back to around 1963 and many of the early ones were OD.
This is what the label looked like back in the mid sixties. The early ones (Vietnam) were not required to have the manufacture's name on them.
My PLs are all retired now, and my replacement is the Kifaru Woobie. But, for the cost of a Woobie, you could get a quality underquilt.
The real PLs are still available as surplus and often the same price as the Chinese copies. Used ones go for less.