Jazzyvee asked on the McVie Artist Showcase Thread yesterday about other tunes John might have played fretless on that wasn't the 'continuously fretted' Alembic... I really had to think about it. Since I'm more familiar with the eponymous White Album than the earlier ones, (and I'll listen through Bare Trees and Kiln House today) the one I have always thought sounded fretless is "Over My Head". Since it's from 1975, this recording pre-dates the stainless steel fingerboard Alembic. Could be just carefully/skillfully played on a fretted bass with flats, because Mac's a genius. Listen carefully for slides on the bridge part-
*I know for sure in later years, he played fretless on this one in live performances. The Dance for one.