I doubt that Manson “discovered” LSD through his connection to the Beach Boys.
SF was awash with LSD (which was ‘legal’ then and pharmaceutical industry produced) when Manson was living there before heading down to LA.
And Timothy Leary was among those experimenting on prisoners with LSD in the early 60s.
"Im the fellow who gave LSD to Brian Wilson in the early Sixties. I have been vilified for doing this for almost fifty years. I would like to set the record straight about this event..."
http://bcb-board.co.uk/phpBB2/viewtopic.php?t=110823#p3102988
On the list of all the things Manson compulsively blabbed about -- "when I first dropped acid" -- seems to be conspicuously absent.