The shirt that Marshall wears throughout the episode says 'Tarkenton Physical Education' on it. Fran Tarkenton was the quarterback for the Minnesota Vikings for most of the 60s and 70s where Marshall is from.
British morning socks do exist, though they're more commonly known as slipper socks. They're a thick pair of socks, with either a double layer sole or non stick material added in a pattern. They're often taken to hotels or overnight stays where you might not want to walk around barefoot on strange (or cold) floors, but don't want to pack bulky slippers from home.