SRAM Chain Types

[wbs]

Does anyone know the difference in a SRAM PC-991 and PC-951 chain. The 951 is listed as 3 grams lighter and is duller/darker than the 991. Is it only a matter of a coating?

[Alex Ramon]

The PC-991 has hollow pins and smoother outer plates, and costs twice as much. As far as I can tell those and color are the only major differences.