160user: No, its not an odd marking. I have two mint, all original, 300 H&H Magnum Bull Guns (one with serial #4,5XX [January 1937] and the other allegedly new, unfired in the crate at serial #535,5XX [September 1961]) that are marked on the barrel exactly as is shown in the linked to photos - "300 Magnum".
Hold into the wind
VarmintGuy
