The red sabot tells me its either pulled from a Barnes 54x50 or its a MMP54x45. The Harvester 54x45 looks a tiny bit different but its really hard to tell for sure. At one time Knight did offer a 54x50 lead but they have not been sold in many years and the sabots would be purple.

It would be helpful if the OP would weigh just the bullet. IIRC Knight had 260gr and 310gr in lead HP...there may have been another weight too. I can look though a 2006 catalog in a minute.