Norma har been loading Weatherby ammo for a long time. MRP used to be their slowest powder, and I believe this is the powder they use in Weatherby factory ammo. The new powder form Norma is the MRP II. It burns even slower than the old MRP. More like Re#25 i suppose. Remember that Norma load their ammo to the pressurelimit. My experience is that ammo made in the USA operate at a lower pressure. Exept Federal High Energy and Hornady Light/Heavy Magnum.
Norway

PS. Norma is located in Sweeden, but the man who started the firm was from Norway.