Chris Willman Senior Music Writer and Chief Music Critic It wasn’t quite a proper Spinal Tap reunion, but there was some implicit history happening Tuesday night at Largo in Los Angeles as Michael McKean and Christopher Guest joined Loudon Wainwright III, the headliner for the latest in a series of Judd Apatow-hosted “Juddapalooza” benefit concerts at Largo. Besides going back together about five and a half decades as friends on the musical-comedy scene, Wainwright, as hardcore fans will know, was seen as a member of Spinal Tap in a cameo in the very first filmed comedy sketch featuring the faux group in the late ’70s. Tuesday’s Largo show featured a healthy intermingling of Wainwright performing songs from his new album, “Lifetime Achievement,” and Guest and McKean bringing back some of the very earliest Spinal Tap “hits,” along with some folk-scene collaborations. Also on tap as unadvertised guests: Beck, who sang a few of his own songs and Neil Young’s “Old Man” along with assisting others in the hootenanny, and top producer Greg Kurstin, who proved game for completely unrehearsed piano contributions and even solos over the course of the two hours-plus.