Not the Dave Mason you are probably thinking of (of Traffic), but the Dave Mason who played keyboards in the first incarnation of Todd Rundgren's side project, Utopia.
At this point, the band was a six-piece progressive outfit, with three keyboard players and album-side-length songs...the kind of stuff Stephen T. McCarthy HATES. They recorded two albums. Another incarnation recorded the Disco Jets album that went unreleased until last year. Later, the band evolved into a four-piece line up that released another eight studio albums and one live album.
They reunited briefly in 2011 as a benefit for one of the other keyboardists, Moogy Klingman, who was battling cancer (sadly Moogy passed away later that year).
Rest in peace, David! If heaven has a place to plug in keyboards, I know you and Moogy will rock the place!