The most popular LP they recorded was in 1959, after all of the original Pioneer Trio had left.

The album called "Cool Water" has a loney cowpoke laying prone drinking from a waterhole, horse in background, hot sun burning down.

This was part of RCA's "Living Stereo" series and was the first SOTP album in stereo.

As far as I can tell, there were six members on the record, probably...

            Hugh Farr (1934–1959) vocals, fiddle
            Karl Farr (1935–1961) guitar
            Lloyd Perryman (1936–1977) vocals, guitar
            Pat Brady (1937–1968) bass
            Tommy Doss (1949–1963) vocals
            Dale Warren (1952–2008) vocals, bass

although...

          Shug Fisher (1944–1959) bass
          George Bamby (1959–1960) accordion

may have helped also.

This record has a fair amount of orchestrated backup and consists mainly of re-recordoings of their best-known material from a couple of decades earlier.