1" Aiming Rifle is a round with the same case dimiensions as the 1" Nordenfelt but fitted with a lead bullet. This was used in the “Aiming Rifle” subcalibre device inserted inside the barrel on the large calibre guns of British Navy ships. As far as I know these were obsolete by the early 20th Century. They earlier marks were fitted with percussion primer and later marks with electric primers.
Shown below is a fired 1" Aiming Rifle projectile.
Here some images:
Yours is the earliest version of the cartridge and does not have these crimps.
OK, thank you. I just want to add that the primer is also a normal one, not a screwed one. Maybe this confirm your idea.
This identification is from Woodin lab.