It is a #1 MK III SMLE, Manufactured by BSA, Birmingham England. (Under King George's reign (hence the Crown with G.R [George Rex - Latin: George King]). A lot of the markings showing on there are arsenal refinish stamps, looks like that rifle has been refinished by the arsenal (Most likely Enfield) several times. Someone has "sporterized" your rifle (obviously).
I had a No 4 Mk 1 that was made by Enfield and had Canadian, British and Australian markings on it (WWII made a lot of these rifles get around the world quickly).
The 18 1/2 tons is verification of proof. The old system of proof was in tons rather than CUP.
You can find some good books for identifying enfields all over the place, some will have fairly complete proof and arsenal marking guide. I am at work right now so I cant browse though them and give you the complete story of your rifle unfortunately.