Actually, depending on when it was made there IS a difference.
Now, from what little I know/have seen I can say this.
I own 2 norinco M14's, they are marked M14, not M305 or M14S. They are both identical to each other, their lettering is stencilled on by machine etc.
I have seen "only" 1 M305. It was purchased probably 20 years or so ago. It is vastly inferior in workmanship/appearance than the norc M14. The lettering is hand etched onto the side of the reciever. And there is no raised portion on the side of the reciever for the scope mount screw, the entire left side of the reciever has a slight cant to it. And those that complain about the norc stocks these days dont know how good you have it compared to the "old" norc stocks.