Loar also signed a rare subset of F5 mandolins called Ferns, of which approximately twenty are known to exist. The name refers to the distinctive fern inlay design of the peghead. The earliest documented Fern bears the serial number 73755, dated July 9, 1923, the same signing date as Bill Monroe's famous Loar. This is the only known Fern built without the "Virzi" Tone Producer, a secondary sound board suspended underneath the mandolin's top inside the sound chamber. This particular instrument is the only known Fern dated on 9 July.