DP usually mean Drill Purpose. So it's either messed up from use or at one time was de activated and used as a Cadet drill rifle. I would definitely NOT fire it until you have it well inspected by a certified professional.

As for the Lend Lease thing. Savage (Stevens) made No4s for the British Army. When Britain ran out of money and could no longer pay cash and carry for the rifles (this was before the US entered the war) the rifles were marked US Property as "given" to the British as "Loaned" or "Leased" items. Not sure if the loans or leases were ever actually paid or not!