The first Grateful Dead show I learned inside and out (or so I thought) was the double-disc One from the Vault, two sets recorded on Aug. 13, 1975 at the Great American Music Hall in San Francisco. Bill Graham introduces the band as the musicians click in one-by-one behind him. Jerry Garcia comes last as Graham’s introduction peaks, “Will you welcome please”—half-beat pause—“the Grateful Dead” and the band drops into “Help on the Way.”

It is, of course, the only time it happened like that. While it’s a truism to say that no two Grateful Dead shows were alike, it’s especially true of 1975, a year that only included four of them. But the Great American Music Hall gig was especially unlike any other Grateful Dead show in that it contained two sets of complex and mature new material buffed to a sci-fi chrome and performed in a 600-capacity room, smaller than the band had played in years. Returning to the recording and the rest of 1975 suggests another truism: It is impossible to listen to the same Grateful Dead show twice.

Once one starts exploring Grateful Dead live recordings, the tendency is to listen to more and more of them, each altering the context of the ones already heard. More than a band with a traditional catalog of studio work and outtakes, no matter how large, the Grateful Dead’s music is a renewable historical resource, changing constantly as new pieces of the past come to light. Even so, it’s possible that 1975 remains the most intriguing year in the band’s ever-intriguing history, filled with creative what ifs and choices that would define the band’s next two decades.

Coming into its 40th anniversary (and the Dead’s 50th), the available documentation pertaining to the band’s so-called “retirement” of 1975 has already been considerable. Besides Blues For Allah, the official product of the year, the ‘75 corpus includes the expected live tapes, a range of memoirs and a cache of rehearsal recordings made at Ace’s, Bob Weir’s new home studio between February and June. Featuring triangular porthole-like windows onto the sylvan side of Mount Tamalpais and a dangerous driveway with roadies loitering at its foot, Ace’s was the Dead’s own private treehouse for the year.