On February 24, 1969, two days before he turned 37, the legendary Johnny Cash led his traveling troupe behind the foreboding walls of the California State Penitentiary at San Quentin, long known as one of America's toughest prisons. Thirteen months earlier, the Johnny Cash Show made musical history when they appeared at another notorious California State pen. The concert yielded what became Cash's best selling album up to that time, Johnny Cash At Folsom Prison, which in July 1968 entered the ''Billboard'' LP chart and stayed on it for 39 weeks, reaching number 13. Johnny Cash At San Quentin, Folsom's follow-up and companion piece, upped the chart-success ante and then some; on August 23, 199 it reached the top of the LP stack in ''Billboard'' and remained there for four weeks, making it the best-selling of all Cash longplayers. Originally a single album, At San Quentin is now a deluxe three-disc, Legacy Edition Package; two CDs containing 31 selections, 13 them previously unissued, plus a DVD called Johnny Cash In San Quentin, a 1969 documentary made by England's Granada TV for British television. It is, by turns, exhilarating and harrowing; among the tunes is a full rendition of ''A Boy Named Sue,'' the rollicking, rowdy smash that in 1969 topped the C & W singles charts for five weeks, while also reaching number two on the Pop side. There are also interviews, some searingly candid, with the prisoners and guards who were present when the Johnny Cash Show packed the big house. Backed by his scythe-sharp band, the Tennessee Three, Cash's quavering bass-baritone renders hits, hymns, history (personal and American), humor and of course, the singular ''bom-chickaboom'' railroad rhythm that is one of Country music's most ineffable, instantly identifiable sounds. The bill also includes first-rate performances from rockabilly king Carl Perkins, the four-part harmonies of the Statler Brothers, and the Carter Family, with bluegrass and country pioneer Mother Maybelle Carter and June Carter Cash, Johnny Cash's wife and the love of his life.