B5IV

JM53, Cowley69, and MK73 list tau Her (HR 6092) as the B5IV standard

Houk88 does not list a B5IV standard. 

# Primary Standards
HD 147394 = HR 6092 = Tau Her (J1619+4618, V=3.9)
  B5IV: JM53,MK73,Cowley69,Jaschek64(most),Lesh68
  B5III:Ahmad52