This show, the second of six at the Capitol Theater in Port Chester, New York, in 1971, has been a trader’s favorite since ancient times, when live Dead music circulated on cassettes. The week began with a drag — drummer Mickey Hart quit after the first night, not to return until 1976. But the Dead had recently made Workingman’s Dead and American Beauty and were on an inspired roll. At Port Chester, they debuted imminent set fixtures like “Playing in the Band” and “Wharf Rat.” The improvising is a bit restrained minus Hart — the long covers of “Smokestack Lightnin’ ” and “Good Lovin'” are rave-ups, not spinouts. But there is a fighting edge to this show — the first-set leap into “Truckin’,” the furious instrumental debate between Jerry Garcia’s guitar and Phil Lesh’s bass in “The Other One” — that settled into a sunny swagger two months later, when the Dead taped the shows on 1971’s Grateful Dead. To be honest, I never played that album as much as my Port Chester tape. Here’s why.