Thank you for the improved photo.
I find it pays to read books that you have had for years.
All is now clear to me at least. Perhaps some already knew the answer. This is a genuine case.
The original British 15 pounder was a BL gun developed from the 12 pounder and both were the Field Artillery standard gun in 1899. These had bagged cordite charges.
News came in 1897 that the French had a revolutionary 75mm QF gun which had recoil and recuperator systems, on a slide principle.
The French at that time were perceived to be Britain's potential main enemy.
We had nothing on the drawing board at that time with anything like that potential.
In 1900 the German public were very anti British, in part due to the Anglo Boer War, not to mention the naval arms race. The British public felt much the same about Germany.
But needs must, so in great secrecy on both sides, we purchased 18 artillery batteries worth of 15 pounder QF guns and equipment, from the engineering firm of Erhardt, Dusseldorf, which obviously included QF ammunition. Every reference to Erhardt was omitted from the ammunition and equipment. These guns also operated on a slide system and had recoil and recuperation.
Your revised photo clearly shows "NK" which is a British marking indicating that the manufacturer is "NOT KNOWN".
Therefore all that I said except the 'K' in the circle stands and was almost certainly Karlsruhe. I also think what I said about the date was right.
In 1907 some of the original British BL guns were converted to QF and were given to the Territorial Army, but the dates would not have been consistent with the Erhardt period.