Hi,
The prevalent theory on these is that 'DP' stood for 'Demonstration Pistol' which is explained in Clive Law's book 'Inglis Diamond'. I think this theory is probably accurate, of the roughly 150 DP series Inglis's made, the majority of them were shipped off to foreign governments for trials and potential purchases. I do have a record that states where the majority of the DP pistols were shipped to, although it is not entirely complete. From what's in the shipping records, it shows a lot of the DP series pistols were sent to Mexico. If you give me the serial number, there's a chance I could tell you where your pistol was sent to.
Here is my 'DP' series pistol that belonged to Tom Clift. Tom came to Canada from the UK during the war to assist with pistol manufacturing at Inglis as an engineer. The pistol was discovered by Toms son sitting on a shelf in a small box at his estate. The pistol appears unfired since leaving the plant, with internals still covered in factory cosmoline.
The consecutive pistol to mine, DP105, is recorded as being part the shipment to the Mexican government.
Regards,
-Steve