“A worn-out old duster. The coat protects your clothing from trail dust, but not much else.”
— In-game description
It is a worn, dark brown, oil skin duster. This type of coat was usually worn by deputy lawmen of the time, likely the reason for its requirements, seen below. It was also a popular choice for bounty hunters.
There is a slight error in the outfit. While it is called the Duster Coat, a true duster would be open in the back, allowing one's legs to be protected while on horseback. The outfit seems more like a hybrid between a duster and a trenchcoat in reality. However, it does affect how quickly dust gathers on Marston's trousers.