I guess if you would check your gvw on the door tag with the info on it if its a dually it should be 10,000 gvw. I have a crew cab thats a dually but recently getting new brake parts I found out that what I should have for my serial number, is not what is installed on the truck. It has 8000 gvw on the door tag. The truck has suff from a xxx,xxx in 1975 and below when the serial number says it should have late 75 or like the 1976 brake stuff. This would be a question for numberdummy how do you find out if your truck was a dually?