Question:~ In the Reader's Digest Record Album, The Best of Gilbert and Sullivan, two well-known D'Oyly Carte singers 'swapped' some parts which they normally played on stage. Who were they?

Answer:~ Donald Adams and Kenneth Sandford.
Pirates of Penzance. Sandford - Pirate King (Adam's stage role). Adams - Sergeant of Police (Sandford's stage role).
Iolanthe. Sandford - Mountararat (Adam's stage role). Adams - Sentry (Sandford's stage role).
Patience. Sandford - Colonel Calverley (Adam's stage role). Adams - Bunthorne and Major.
The Gondoliers. Adams - Don Alhambra (Sandford's stage role). Sandford - Duke of Plaza-Toro.
The Mikado. Adams - Ko-Ko.