I believe these are symbols of martyred apostles.
- Key: Peter
- Sword: Paul
- Axe: Possibly Matthew
- Trident: Thomas
Jacob Prahlow has collected some research.
Although an upside-down cross is associated with Peter, he was given the keys of the kingdom (Cf. Matthew 16:19) and the key is a very common symbol.
Paul, a Roman citizen, was executed by the sword rather than crucified. While the date of his death is rather hazy, the place is known and there is an enormous basilica on the site. Paul is very often depicted with one or two swords: the sword of truth and the sword of martyrdom.
Both Matthew and James brother of John are traditionally held to have been martyred by beheading. James is recorded in Acts 12 as being killed “by the sword” — Matthew’s death is not recorded, but one tradition is that he died by the axe.
One legend of Thomas’ death in India is that he was killed with a trident.