If I read right, its the '84 version of the M1973. Without the Serial Number I cant say when it was made. You should find that out.
It the later life of the M '73s the Army was starting to use smokeless powder in the 45-70. The smokeless powder loads never were fully adopted because the Army went to the 30-40 Krag.
I have a later model with the newer sights. The sights were set up for the 405 gr bullet with a velocity between 1200-1300 fps.
A good safe smokeless load for the trapdoors is 28 grs of 4198 pushing the 405 gr. bullet. It's right with the black powder pressures, the same 1200-1300 fps velocities, and it matches the sight marks on my rifle.
What's more, its a pleasure to shoot. In my old age I have become a wimp when it comes to recoil, yet I can shoot the above load, (with the rifle's steel buttplate) all day without discomfort.
The max. pressure suggested for these rifles is 28000 psi. Per quick load, the above load develops 12500 psi.
Lyman's reloading manual list the 31.5 grs of 4198 as the min. load for the Trapdoor, @12400 PSI, but I've found the 28 gr. load comes closer to matching the original 405 BP load. It does match my sights better.
Remember, the 45-70 was originally the 45-70-500, or a 45 cal bullet, pushing a 500 gr. bullet with 70 grs of black powder for the RIFLE.
The carbine load was 45-55-405, or a 405 gr 45 cal. bullet, pushed by 55 grns of BP.
Late loading the army adopted the 405 gr for the Rifle, Carbine and Cadet rifle.
Of course, this all depends on the condition of your rifle. If in doubt I'd recommend using the 55 gr. BP load, Making sure with black powder, the bullet is seated firmly on the bullet.