I believe you are correct, and I've corrected my earlier post. As I understand it now, the "N" stamp signified a slightly large bullet neck, and rifles not with the "N" stamp would have difficulty closing the bolt on this cartridge. My source indicate favorable to fire from the Hotchkiss machinegun as well given the heavier bullet weight.
I heard that the red varnish of these cartridge specifically designated such cartridges for the Hotchkiss and thus they were mostly found on the gun feeding tray. Loose rounds being pulled out of these trays later and sold in batches or individuals. I have not confirmed this and I must say I do not believe it holds much truth. There seems to be more correspondence between color/bullet weight rather than color/cartridge/gun use.