Possibly someone smarter re: British proof marks could clarify, and thank you in advance.
Under the 1896 Rules of Proof, the maker could request Supplementary Nitro Proof with either "E.C." or "Schultze". I don't know when that was permitted prior to 1896
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA2&dqStandard powders used for proof were at the discretion of the Proof House, and were given as T.P. (Tower Proof) “which shall be of strength equal to Waltham Abbey “R.F.G. 2” and of grain varying between Nos. 4 and 5, that known as Curtis and Harvey's “T.S.” (Treble Strong) No.2 [for Supplementary Nitro Proof with 4 1/2 Drams and 1 2/3 oz. “soft” No. 6 shot] and, that known as “Col. Hawkers Duck Gun Powder”, and the nitro-powder known as Cordite, or any other description of nitro-powder which may hereafter, from time to time, be adopted by His Majesty's War Dept.
I have not seen a British proof mark indicating proof with "E.C." or "Schultze" as was used by the Belgian proof house, nor is one shown here
https://books.google.com/books?id=lvBk8df8PjUC&pg=PA139&lpg Or this mark indicating proof with E.C. No. 3, a 33 gr. = 3 Dr. Eq. powder
1896 - 1904 some British guns did have the Service Load marked, like this 1896 Lancaster.
42 grains of the original "E.C." Bulk Smokeless was 3 Drams, with 1 1/8 oz. shot
or the MAX. shot charge, but without the grains Nitro powder