I owned Oasis #3 for several years. Gary Cooper built #1 for himself, then #2, Kool Bell's bass, then my guitar (maybe 1974?). It was stereo, with Hi-A pickups, 2 tones, a volume, and a master volume. Gary's wife's family was from Roseville. He left Alembic and they moved up there, where Gary started Oasis in Sacramento. I think it lasted about 5 years. After that he was a guitar tech at Skip's Music in Sac for about 30 years. He was a good guy to visit, always had lots of good Dead stories.
So I guess Gary worked at Alembic in 1973, at the beginning of the Wall of Sound time. As most of us know, Alembic was a well established company by then.