In the play 12 Angry Men, what assumptions are made by the jurors after the knife is produced and how does the 8th juror counter their assumptions?

When a discussion about the murder weapon, which was identified as the knife purchased by the defendant, a “one-of-a-kind” knife, begins, 8th Juror surprises the others by presenting an identical knife he had purchased in a pawn shop two blocks from where the boy lived a few nights prior, shattering the claim that the knife was so unique and identifiable.