AC/DC played the final show of their obstacle-filled Rock or Bust tour last night in Philadelphia. Longtime bassist Cliff Williams, who had previously announced that he will be retiring from the group following this show, received a special salute from guitarist Angus Young at the end of the set.

As you can see in the video above, as the band wrapped up its traditional closing song, "For Those About to Rock (We Salute You)," Young grabbed Williams from his customary spot at the back of the stage and brought him to the front catwalk, allowing the audience to show its appreciation for the bassist's nearly 40 years with AC/DC.

As the crowd roared, Williams humbly retreated to his usual position. So Young turned his attention to singer Axl Rose, who took over as frontman for the tour after Brian Johnson was forced to (at least temporarily) retire because of hearing damage.

Young playfully knocked Rose's hat off his head, then offered quick acknowledgements to his nephew Stevie Young (who replaced Angus' ailing and now retired brother Malcolm on rhythm guitar prior to the start of the tour) and drummer Chris Slade, who rejoined the group following Phil Rudd's much-publicized legal troubles. (As we noted earlier, and as you can see in the gallery below, it's been a challenging tour.)

Since Rose took over as singer, AC/DC's nightly set lists have ballooned from 20 to 24 songs, with an added emphasis on cuts from their early Bon Scott-fronted years. The crowd in Philadelphia got yet another bonus last night, as the band broke out "Problem Child" for the first time on the tour just before the show-closing "For Those About to Rock."

Watch AC/DC Perform 'Problem Child' in Philadelphia, 9/20/16

It's possible this could have been the final AC/DC show ever, though sole remaining original member Angus Young recently stated that he doesn't know what the future holds for the group.