The String Pak Butterfly was an unusual Duncan butterfly model: you could place two or more spare yo-yo strings (or "secret messages" as Duncan describes) inside the cavity of each halve. Both halves have snap-on side caps that you could easily remove to access the spare strings.
An uncommon and short-lived Duncan Butterfly model that was first released in 1979 during Duncan's 50th anniversary celebration.
Of interest... See the Duncan 1979 product catalog that introduced the String Pak Butterfly yo-yo for the first time!
