Possible Clues:

• Francis Drake and Ernest Shackleton, for two
• Men with knighthoods
• Dated letter salutation
• Lancelot and Gawain of legend
• Ian McKellen and others, title-wise
• Lancelot and Mix-a-Lot, e.g
• Opening of some formal letters
• Titles for baronets
• Dubbed gentlemen
• No-longer-common letter opener