It's not the funk-rock Holy Grail, but By Way of the Drum, a trippy gem from Funkadelic, is so rare, even George Clinton doesn't own it. ''If you can find one, send me a copy,'' he says. ''I haven't heard that in 15 years!'' In the early 1980s, Clinton signed a deal with MCA Records, which expected catchy electro-funk in the vein of his '83 hit single ''Atomic Dog.'' What they received instead was a fusion of head-tripping psychedelic rock and body-shaking drum-machine beats. (A funked-up cover of Cream's ''Sunshine of Your Love'' is particularly out-there.) MCA rejected the album with a brusque letter: ''We would like to inform you that we will not be releasing the album by your 'so-called' concept group, Funkadelic.'' The master tapes have gone missing, but some hardcore Funkadelic collectors treasure hissy cassettes of the surprisingly sweet set.