Recorded in April-May of 1968 and released in August of that year, the Byrds' "Sweetheart of the Rodeo" album was ahead of its time, foreshadowing the blend of country and rock that became an integral part of American music in the years and decades that followed. Its influence was profound enough to warrant a 50th-anniversary tour, which brings Byrds co-founders Roger McGuinn and Chris Hillman to ACL Live on Saturday backed by Marty Stuart & His Fabulous Superlatives.

McGuinn and Hillman, who teamed with Gram Parsons to create the "Sweetheart" album, naturally are the key figures here. But in some respects, the perspective of Stuart serves as a linchpin that brings all of the history together.

Let's travel back to May 11, 1973. Stuart is a 14-year-old mandolin prodigy playing with bluegrass legend Lester Flatt, who's booked on a bill at Michigan State University with the Eagles headlining and Gram Parsons and Emmylou Harris opening. Stuart had recently bought the "Sweetheart" album.

"That night, I saw honky-tonk and gospel and country music and rock 'n' roll and folk music collide successfully," he said. "And I went, 'Oh, this is how to live a life, right here.'"

Fast-forward 45 years. It's Feb. 16, 2018, and Stuart is in Austin for a show with his band at Antone's. While here, he gets a phone call from McGuinn, who's at an airport in Buenos Aires, Argentina, and has an idea. Would Stuart and the Superlatives be willing to join him and Hillman on a 50th-anniversary "Sweetheart" tour?

"I sent Roger the word back: 'Whatever it takes to do this, the answer is yes.' I jumped on this one because it just felt right."

McGuinn had worked with Stuart occasionally over the years, including on a film production in Nashville. During breaks, McGuinn recalls, they'd go down to a creek with a couple of wooden folding chairs and do some picking. A key connection: Stuart now owns the Telecaster guitar that belonged to the late guitarist Clarence White, who played on "Sweetheart of the Rodeo."

Stuart, a mainstream country star in the late ′80s and early ′90s, shifted gears to a broader Americana sound after the turn of the century, recruiting a band that's as good as any in the business. The playing and singing talents of guitarist Kenny Vaughan, drummer Harry Stinson and multi-instrumentalist Chris Scruggs make the group "a perfect fit" for the current tour, Hillman says.

The band previously had done some touring with McGuinn, and “he realized that when Marty backed him on our Byrds stuff, it was so good," Hillman said. "They're such good players, and they love the music."

IT'S THE REUNION of McGuinn and Hillman that makes this tour so special, though. "I haven’t worked with Roger in decades on anything extensive like this," says Hillman. "I’ve been having more fun than I've had in 40 years."

From his spot alongside them onstage, Stuart can see it, too. "I’m watching Chris and Roger go to a different level of their relationship," he says, "and it’s just a wonderful thing to be a part of."

The backstory of how and why the "Sweetheart" tour happened is bittersweet. When Hillman was last in Austin a year ago this week, he was touring to support "Bidin' My Time," a beautiful album he'd recorded in early 2017 with Tom Petty. The loss of Petty in October of last year was a tough blow for Hillman and also McGuinn, who'd first been a mentor to Petty and later a close friend.

Hillman had considered canceling the rest of his tour after Petty's death, but advice from McGuinn helped persuade him otherwise. "I called up Chris and I said, 'You know what, Tom wouldn’t want you to quit. He’d want you to do it as a tribute to him,'" McGuinn recalled.

After the tour, Hillman had another rough blow when wildfires raged through his home community in Southern California. "Chris went out to dinner on his birthday and came home to find his house on fire," McGuinn says.

A few weeks later, McGuinn continues, "my wife and I were sitting in the airport down in Buenos Aires, and we said, 'You know, we should do something nice for Chris. Let’s put together the 50th anniversary of 'Sweetheart of the Rodeo' tour, with Marty and Chris. That was the reason it came together. It was just a labor of love."

THE ALBUM'S 11 TRACKS are the centerpiece of the show, but they're far from the only attraction. A nine-song set before an intermission primes the crowd for what's to come, connecting the dots from the Byrds' early folk-rock days with songs such as Porter Wagoner's "Satisfied Mind" and the McGuinn/Hillman co-write "Old John Robertson."

“The first set is designed to show that we were into country music before we went to Nashville and recorded the 'Sweetheart' album," McGuinn says. "Chris came from a bluegrass background, and I came from folk music, where country music was a part of folk music.

"And then Gram came along. He was the big spark plug; he got us all excited about doing a pure country album. I went out and bought a Cadillac Eldorado and cowboy boots and a cowboy hat, and I used to drive around L.A. listening to Dolly Parton and Porter Wagoner. And I thought, 'Man, this is great stuff; we've got to do this.'"

It wasn't the first time the Byrds had shifted gears. "We started out by interpreting Dylan with a Beatles beat, and that became folk-rock," McGuinn says. "And then we experimented with John Coltrane and Ravi Shankar, which was misunderstood to be psychedelic, but it really wasn’t intended to be. And then we did something about as far from psychedelic as you can get: We went to Nashville and did a country album. So we were just trying to keep from getting locked into a box.”

The Nashville sessions included pedal steel guitarist Lloyd Green, who was in attendance last month when the "Sweetheart" tour packed Nashville's historic Ryman Auditorium. The Byrds made a now-fabled appearance there in 1968 on the Grand Ole Opry: Scheduled to play a Merle Haggard song, they did Parsons' "Hickory Wind" instead.

"The audience was less than enthusiastic," McGuinn recalls wryly. "I don’t specifically remember this, but Lloyd Green said they booed us."

Another side musician on "Sweetheart" was pianist Earl Poole Ball, who now lives in Austin and plays regularly with local country group Heybale. Ball, who lived in Southern California at the time, was among a handful of musicians who laid down tracks for the album back in Los Angeles.

"I was lucky that they called me," Ball said last month. "I think it was my first real rock 'n' roll session on a big budget." He'd recorded with Parsons previously on an album by the International Submarine Band, but mostly he worked with hitmaking country acts such as Buck Owens.

"They weren't out to get three songs in three hours like I was used to doing at Capitol Records" with Owens producer Ken Nelson, Ball continued. "Over at Columbia, it was just altogether different."

THE ENCORE SONGS of the "Sweetheart" shows in some ways bring the tour full-circle back to its initial inspiration. After playing "So You Wanna Be a Rock & Roll Star" — a McGuinn/Hillman co-write that McGuinn recently re-recorded on his new solo album "Sweet Memories" — they hit a three-song stretch that pays tribute to Petty.

First comes "King of the Hill," a song McGuinn and Petty wrote together in Sweden in the 1980s while on tour with Bob Dylan. A demo of the song is included on "An American Treasure," a posthumous Petty box set released a few weeks ago.

They also do "Wildflowers," a Petty classic that Hillman recorded on his "Bidin' My Time" album last year. And Stuart's band — which worked with Petty's Heartbreakers bandmate Mike Campbell on their 2017 album "Way Out West" — gets in on the act with a revelatory version of "Runnin' Down a Dream." "Marty plays mandolin, and it’s phenomenal," Hillman says. "It’s all acoustic, but it just works."

The tour is, perhaps, Petty making good on one last promise to Hillman, even from the great beyond. “The last conversation I had with Tom Petty was in his house," Hillman says. "And I said, 'Well, this is the last album I'll probably ever do, and you really did a great job; you got great performances out of me.'

"And he says, 'What are you talking about? I'm not done with you yet.'"