It might mean that he'll be playing the harp tho.
I mean, Korenaga, who is probably gonna be doing vocal and guitar, is credited as (G/Vo) so (B/Harp) should mean he's playing bass guitar and the harp?
Music by TAMATE BOX(FUZ)
Arranged by TAMATE BOX&Shuho Mitani
Piano by Akinori Handa
Electric guitar&Acoustic guitar by Shuho Mitani
Acoustic guitar by TAMATE BOX
Bass by Sakurai Rock
violin by Hitoshi Konno
viola by Ayaka Jomoto
cello by Shinichi Eguchi
Recorded&mix by Jun Shoji