The first album, Surfin' Safari, was mono and Duophonic.
All the 1963-64 albums (Surfin' USA through Christmas Album) received dedicated mono and stereo mixes.
Beginning in 1965:
Capitol did a stereo mix of The Beach Boys Today! and Brian rejected it. That stereo mix is believed lost.
All the Beach Boys' 1965-1967 albums (Today through Wild Honey) were mixed only in mono, and the stereo versions were Duophonic.
Friends (1968) was the first new Beach Boys album mixed in stereo since the 1964 Christmas album.
Hope this helps.