Thanks s2503000. I sent an email to Accurate and got a reply quickly.

Caliber: .450 Marlin.
Barrel: 24”
Powder: Accurate – 1680®.
Bullet weight: 325grains.
Start load: 44.6 grains (1875-1975 Fps)
Maximum load: 49.5 grains (2100-2200 Fps).
Bullet weight: 350grains.
Start load: 43.2 grains (1825-1925 Fps)
Maximum load: 48.0 grains (1975-2075 Fps).

NOTES:
It’ important to note that SAFETY is our prime concern therefore we strongly recommend.
1. ALWAYS BEGIN LOADING AT THE RECOMMENDED MINIMUM “START” LOAD and develop loads in 2% increments towards the MAXIMUM load.
2. CAUTION: Beware of double charging if the loading density is below 50% of the available volume.
Examples:
Ø Most Handgun caliber/powder combinations as well as
Ø A-5744 with reduced loads for rifles.