I have a 1972 vintage No.1-H in .375 and just got a new 1-H in .405 Winchester. Barrel contour is the same, but there is a slight difference between the two in the shape of the action, specifically where the forend fits against the small extension at the lower front of the action body. Other than that, either forend would fit either barrel. I don't know when the action was changed, or why.