The .577 3 1/4" was offered in BPE, Nitro for Black, and full Nitro. The Nitro version was offered as both 90 Cordite/650 bullet and 100 Cordite/750 bullet.

I think I know which rifle you're referring to. Just "577EX", no mention of bullet weight or charge and no mention of Cordite. That makes the proof marks pre-1904 black. Serial number is from a block assigned 1898-1901. Nitro proof was around long before the 1904 rules introduced the nitro proof mark. Under the 1887 and 1896 rules, nitro proof was signified by marking the name of the semi-smokeless propellant used together with the heaviest bullet used in the normal load. For guns proved in Britain during this period, if a semi-smokeless propellant is not named on the flats, it wasn't proved for or with nitro. This gun was built to full nitro weight, but it isn't nitro proof.
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