Now we finally have good photos! No, of course it doesn't say USA. It's a crown over 9 over A, and is a perfectly normal Britishacceptance mark.
After the necessary cautious comments by Col. Colt and myself, it is a relief to say that you have a good P13 type bayonet, made for the P14 rifle, which has become paired with a P13 scabbard as made for the M1917. Needless to say, it is now impossible to say how they came together. But as there is NO factory overstamping with US and cancellation of the British marks, we may assume that it did NOT happen at the factory.
The moral of this thread: First get good pics, and then ask the questions!
Patrick