Bugger

, just spent an hour detailing the stamps then lost it before I posted it.
So here's the short version. Hope it helps fill in a few gaps Candyman.
Crown over initials (GR) over crossed flags over P = proof mark.
Crown over number over letter = inspectors mark. E = Enfield. Italic B = BSA
' + 2 numbers = date ('37). Usually indicates year of inspection, refurbishment, rebarreling.
What looks like a D with a horizontal bar on the upright is a combined E,F & D an Enfield marking.
.303, 2.22, 18.5 and an arm holding a sword over NP = commercial proof mark.
England - Import/Export stamp, applied to denote country of origin. Can't recall if applied in the UK or US.
Arrows - british property mark.
NS - nickled steel. Also M & U - denotes different metal compsition specification.
Crossed pennants - proof mark.
A few othjer thoughts -
the sling swivel on the front of the trigger guard was used in early MkIII's.
Note the slightly different fonts for the serial numbers on barrel and receiver. The font on the bolt appears a closer match for the barrel. Replaced at same time ?
III on the safety is common, have not seen it on the bolthead before.
The W in star appears to have been overstamped with a circle (unlapped barrel) but also appears to have something in the centre (JJ ??)
I incorrectly said a stamping was on the seer, it's actually on the magazine catch - brainfart
Here is a shot of a magazine cut-off in place as mentioned by Bradtx