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I've got one too and as far as I know it is something like "Drill Emergency" which means it might not be top grade and, militarily, to be used for Drill purposes (unless you have to use it)
According to Skennerton's book, page 491, it means that it was built at Enfield and was the mark applied, among others, to No4 Mk1 Trials rifles and (T) snipers.
DP means Drill Purposes only, and not for use with live ammunition.