For the early part of the Beach Boys career, all of their singles were mixed and mastered and released only in the mono format - they didn't release a single in stereo until 1968. In those days, hits were made on AM radio in mono. And the mono of those times worked well for Wilson, who suffers from partial deafness. Still, apart from Surfin' Safari, which was mono only, the Beach Boys did turn in stereo masters on all of their early albums up until 1965. The Beach Boys Today! was the first LP for which they only delivered a mono master.