All I wrote is: before 1904 a "Nitro"marking on a rifle that was proofed for nitro cartridges was not necessary in every case. FI my 1899 vintage London proofed Gibbs Mannlicher is not marked "nitro" or "cordite" either. It simply was not necessary! Other than many other British cartridges, including the .303, there never was a blackpowder factory load in the 6.5x53R aka .256 Gibbs Rimmed case. So it was completely unnecessary to apply such a mark, as it was impossible to have a rifle so chambered only proofed for a blackpowder factory load. The same applies to the .400-360 cases.
BTW, original commercial Oberndorf Mauser rifles of pre-WW1 days are also lacking both the crown/G (rifled)and the crown/N (Nitro) stamps. Simply there were no M98 sporting rifles not rifled or not made for smokeless loads.