The majority of the Windsor-made P53 Rifle Muskets were sent to England in fulfillment of the contract with the British Government, but some did stay here as they became surplus to British needs at the end of the Crimean War. They were proofed here in the US in order to assure that they would pass proof in England but those getting to England did have to pass proof there there as well. What proof marks were applied here, if any, I have no idea. I think, but I may be wrong, that they were proofed in the London proof house for British Government acceptance. For resources, you might try:
The British Soldier's Firearm - From Smoothbore to Smallbore 1850-1864 by Dr. C.H. Roads
(Published by R&R Books, 1994 (first edition 1964), ISBN: 1 884849 13 X)
The best online source will be David Minshall's "Research Press" at:
http://www.researchpress.co.uk/Particularly:
http://www.researchpress.co.uk/firearms/british/enfield/index.htmand this in particular:
http://www.researchpress.co.uk/firearms/british/enfield/markings_p53.htmThe above link has a short explanation of the lock markings on the Windsor made P53 Rifle Muskets but you will need to go deeper for an explanation of the proof marks on the barrel.
Out of curiosity, are you doing this for a reenactor? Be careful with how far you go, many in the so called "authentic campaigner" movement will try to insist on authenticity of markings to the point of downright counterfeiting in the opinion of firearms collectors and historians. Nothing wrong with it in my personal opinion, only those who are minimally informed and do not do their research before buying will get taken in by a reproduction, but it can reflect badly on you. They tend to look at firearms as props, not collectors items and have no concern with the collector community. Harmless really, just a different way of looking at things.
An interesting fact, the British felt these were arms of the Second Class though they were well made and passed all of their inspections.