I'm not a "Remington expert", but from what I've been told, 1975 was the last year of the chrome carrier. I have one made in 1972, and it has the chrome carrier. I love the 870 Wingmaster though. Best shotguns ever made. Mine is a lefty..
In this picture, you can see the chrome carrier on the left hand 870 Wingmaster, and the blued carrier on the 1979 wood stocked Wingmaster.