Hi. It's a 'bubba'd' Short Lee-Enfield(SHTLE) No. 1 Mk III*. The receiver was made in the Royal Small Arms Factory Enfield in 1916. Thousands of 'em were assembled out of parts bins with zero QC. Do not shoot it until you have had the headspace checked. Headspace was adjusted in the factory by changing bolt heads until the headspace was safe. If the headspace is bad, you have an issue. The only way to fix it is to try multiple bolt heads with proper headspace guages(no empty cases, live ammo, bits of tape or anything else.) until you find a bolt head that gives safe headspace. No. 1 bolt heads run $13.30 each for a stripped one(you take out the extractor and its spring then put 'em in the new bolt head. Not a big deal.)from Gunparts.
You should slug the barrel as well. Hammer a cast .30 cal. bullet or suitably sized lead fishing sinker through the barrel and measure it with a micrometer. Lee-Enfield barrels can measure from .311" to .315" and still be considered ok. Over .315" the barrel is shot out. Commercial hunting ammo uses .311" or .312" bullets. Accuracy will be poor through a larger barrel.
"...303 8? TCNS..." That'd be BNP(Birmingham Nitro Proof) .303 2.222" 18 tons plus either / SQ " or per Sq ". The cartridge, its length and the pressure it tested at. 18 tons per square inch. Proofing stamps required by British law on any milsurp sold through England. You'll see 'em on M1 Rifles too. Nothing to do with Lend/Lease though.
The 'England' stamp is there to comply with pre-GCA of 1968, U.S. import laws.
The 'G. R.' means George Rex. As in King George V of England.
The Crown GR, Large X(actually crossed flags), P is a British proof mark.
"...Left rear barrel:..." Hard to tell from the description, but it's likely a proof mark too. Or an inspection mark.
The crown with the 'E' and the number is an inspection mark. The number indicates a specific inspector.
"...graduated to 20..." 2,000 yards. During W.W. I, the PBI were used as a sort of artillery using volley fire. Literally plunging fire with a whole battalion's rifles at very long range.
There's lots more info about stamps, etc., here. Inspection and proof marks didn't change much between the No. 1 and No. 4.
http://www.enfieldrifles.ca/main.htm
There's a how-to for disassembly/reassembly here. Right colume.
http://www.surplusrifle.com/smle/index.asp
