I have no idea why Greece bought 7.9 ammunition from DWM in 1939, but I do not think this was the ammunition supplied to England by Greece. The DWM headstamp for Greece seems to be very rare. I had never heard of it until this thread, and I had a pretty fair “Greek Collection” of 7.9x when I collected that caliber.
The ammunition supplied to England, I believe, was that with the P C H 39 and 40 headstamps and with PC 39 IZ and PC 40 IZ headstamps. These are fairly common rounds found in ball with purple primer seals, Tracer with red primer seals and AP with green primer seals and a green tip. These are all British ID colors. There is even an AP with a W added in front of the “IZ” marking, evidently done in England after the fact.
I cannot provide photos as I no longer have the cartridges. The information is from my collection catalog of 7.9 x 57, which I retained as part of my library when I sold the collection.