Not mentioned is that there are at least 4 versions of the link couplers. 1946 production with a narrow shank (and a matching pin), 1947 production with a thicker shank, but still no weight. 1948 production with a brass weight added, and then 1949 to 1953 production with a black weight. Pins go from nickel plated to cadmium plating too.
I assume you know that that red 631 is one of the harder gondolas to find. If not, I'll trade you a green 931 straight across and then you'll have another Knuckle coupler car!!