Possible Clues:

• "Who knows?"
• "Who knows?!" (In this answer, note letters 10-7)
• 'It's always a bit of a mystery'
• Shaw play? I'm not sure
• It's impossible to know, so safeguard that information always!
• Who knows? Take that secret to your grave!
• Doubt expressed, perhaps, that marksman will succeed in opera
• Play translated eventually, once first of revisions is accepted
• George Bernard Shaw play
• Play without determination?