Lens serial numbers

I have already seen a message earlier that says that a USA 50mm f1.8 lens has US XXXXXXX after it....

To my knoledge this has never been done before as all the lenses are coming from the same factory, why should they have US on I really can not explain..

Go and look at one, I am curious..

I am sure that EU lenses do not have EU 1234567 on