Possible Clues:

• Half a round
• They're all round but only half round
• They're drilled only half way round
• Muses, then scrapes enough for half a round?
• They're drilled only half round, of course
• All round half way round, of course
• Being all round, they are only half round, of course
• All round, but of course only half round
• A quick half?
• They can be the back or the front