To answer my own question, Barry's track "One by One" was included on Horizon's album "WP-1636/SWP-1636 - The Barry McGuire Album - Barry McGuire with the Horizon Singers [1963] One By One/Doo Dah/Town And Country/The Way You Are/Greenback Dollar/Little Boy//So Long Stay Well/Banjo/Another Country/The Good Times Is All Done Now/Oh Miss Mary/Old Paint"
In 1965, in the wake of Barry's success with "Eve of Destruction" tracks from two Horizon albums were then repackaged into:
Surrey SS 1003 - Star Folk with Barry McGuire Featuring Members of the New Christy Minstrels - Barry McGuire [1965] Compilation from Horizon WP-1608 and WP-1636. Greenback Dollar (S)/Far Side Of The Hill (M)/Fireball Mail (S)/Ride Around Little Doggies (S)/Stay Long Stay Well (S)//Puff The Magic Dragon (S)/The First Time (S)/Gold Wedding Ring (S)/One By One (S)/Oh Miss Mary (S)
The track "Town and Country" with the B-side "One by One" were released as a single.
The tracks are all available online for purchase.
I'll note that Barry covered "Oh Miss Mary" a classic Journeymen tune by John Phillips.
Dave