Possible Clues:

• As old prophet might put it e.g., 'O Thou! -- whatever title suit thee ...'
• Sign of omission made by guy perhaps consuming hour on a job
• What greengrocer's offering here with unusual accuracy?
• Address to person not present that should be in St Paul's?
• It's mark of ownership
• Part of winner's prize shortly to be received by a model
• Mark similar to 18, but higher
• Higher pair of black squares in this grid, typographically
• Sign of letter dropped in the post, or a dodgy 'p' for 't'
• Guardian's printers use this, perhaps too loosely