Released in 1978 when both Waylon Jennings and Willie Nelson were enjoying massive popular success after decades of toiling as journeyman musicians, Waylon & Willie finds these two iconic “outlaws” performing straight-forward unadorned country music. Much of the industry had turned to gratuitous female groups and orchestrated string sections to make the country sound more palatable to mainstream audiences, but Jennings and Nelson preferred the rustic charms of acoustic guitars, harmonicas, and pedal steel guitar. “Mammas Don’t Let Your Babies Grow Up to Be Cowboys” turned into their biggest hit and it’s one of several duets here. Two Kristofferson songs — “The Year 2003 Minus 25,” “Don’t Cuss the Fiddle” — sport a lighter tone than you might expect from outlaws, while Jennings’ take on Fleetwood Mac’s “Gold Dust Woman” features a charmingly dated phased-out electric guitar chasing a modest funk. Nelson is at his wistful best for “A Couple More Years.” Though hardly as landmark as such a meeting of two industry veterans might suggest, it’s a sturdy snapshot of that bygone era.