I have not finished up the canister of Unique that my father acquired sometime before 1965. I earlier asked if anyone else was using a powder that old or older. If I remember correctly no one else admitted it.
The Unique was originally in a paper and cardboard canister, it wasn't till about 20 years ago that I transferred it to a mason jar. Still using it for light loads in my 45s for snake shot and with some plated hollow points for varmints.
If your 2400 was not stored in direct sun and under reasonable temperatures it should be alright. Your metal container will be reasonably air tight, my paper canister was not. I made the mistake of using the 2400 for some higher pressure loads in a 30-06 with 100 and 110 grain bullets. It did make a ring at the point the bullet bases were and actually enlarged the chamber at that point. My solution was to set the barrel back .200 inches to get past that ring with the extraction problems. If you use it in your Krag with lighter loads and cast bullets you should not have that problem.