Fifty-one years ago this past weekend, a crowd of half a million converged at Max Yasgur’s Farm in Bethel, New York for the Woodstock Music and Art Fair. The four-day event became a cultural touchstone and mythical ideal, strengthened even more by the Woodstock film and soundtracks that proliferated the marketplace during the early ’70s and beyond. While performers, fans, and countless supposed attendees participated in even more myth-building, a small group of historians and researchers worked to piece together what really happened. Last year, a decade-long dive into Woodstock tape vaults…