Paul, I spoke back to Horst Thielbörger who wrote the book on European flare gun cartridges.
He told me the following:
This is a flare gun case (basing on the 1" Nordenfelt). The half-knurled rim is indicating a green signal.
As for the user there are two possibilities.
The likelier one is that this is the first production year for Norwegian flare gun cases (as Norway became an independent state in 1905). These were shortly made before Norway switched to 28mm (by making the Nordenfelt case straight by omitting the last draw for the case where the conical shape was formed).
The less likely but still possible is that this was made by Raufoss for Sweden who only later switched to a new case design of a straight 25mm case which was not related to the Nordenfelt anymore.
So what looks like a projectile here is a self made thingy (steel makes no sense anyways).
The case itself is a very scarce item.