TIA:
If it is on the cartridge, it is could be for "Normal" like 9.3X71 Normal although it is not the same as 360 2 1/4" Express - 9.3X57.4R. DWM 292 is a 9.4X57.5R Musket, so the "N" might be an abbreviation for something similar. Are the cartridges current production or are they reformed cases with a parent headstamp? Could you post a pic?
Nickled rimmed cartridge case and flat nosed jacketed soft point bullet.
Headstamped "N" at 12 o'clock, with "360-8X57" from 8 to 4 o'clock.
By the looks of it, not a recent loading. I'll try to get hi-res photos when I'm home again.