I think You need to get the UK dealer to submit it for dating by an expert recommended by Defra to obtain legal "cities" recognised international documents.
Runjeet on this forum from Akaal arms in the UK can probably advise you well on this.
or indeed provide the expert opinion for the required documents.
Also Don't ship through New York,New Jersey or California...
Even with documents, my understanding is it would be illegal in those states..
spiral