According to Juror #4, the boy left home at about 8, after getting into a fight with his father. What did he buy, and what did he claim happened to his purchase?

The 4th Juror explains that, on the night of the murder, the boy bought a uniquely carved switchblade knife identical to the one used in the murder. The boy claims that he lost it that night, before coming home to find his father dead. 4th Juror presents the death weapon, the “only one of its kind".