Top billing had four noted roles, one more than the three the billing allows. The Prologue and Quint are the same singer here (Peter Pears) - Decca 475 6029 booklet p.18, scanned in rips/rip-1618/Scans/Britten2 - Notes09.jpg, lists Pears against both - so dropping the Prologue leaves Vyvyan, Pears and Mandikian billed and changes nothing that is displayed.