Possible Clues:

• Fastener designed to leave a flush surface
• One may be near a washer
• Letter-shaped metal fastener
• Screw-on fastener
• Fastener with a letter-shaped cross section
• Flush fastener
• Type of bolt holder
• Item listed above "u-bolt" in a hardware glossary?
• Item listed above 'u-bolt' in a hardware glossary?
• Model airplane fastener