LIKE ANTHROPOLOGISTS searching for the "missing link," musicologists have long sought the record that would illustrate the connection between pre-World War II old-timey string-band music of the Appalachians and the post-war, honky-tonk country music of the Mississippi Valley and the Southwest.

Though it was only recorded last year, Hazel Dickens' new album may serve as that missing link. Dickens is backed by an all-acoustic string band of the best bluegrass pickers in Nashville, but she sings with the tougher-but-wiser realism of the honky-tonks.

The album's title, "It's Hard to Tell the Singer from the Song," sums up Dickens' heartfelt identity with her material. When she sings about the "Hills of Home," it's with a knowing ambivalence that's far more convincing than John Denver's valentines to the same geography. When she asks a lover, "Do Memories Haunt You?", the reluctant catch in her voice makes it clear that memories haunt her. When she tells another ex-lover that "You'll get no more from me," the steeliness in her tone warns him against even trying.HAZEL DICKENS -- "It's Hard to Tell the Singer from the Song" (Rounder 0226). Dickens appears with Mike Seeger, Kevin Roth, the Fiddle Puppets, Cathy Fink and Marcy Marxer in a benefit concert for ailing Appalachian singer Ola Belle Reed Friday at Blair High School in Silver Spring.