I know not from whence it came.
It came from Fishmonger’s Hall - the headquarters of the Worshipful Company of Fishmongers, one of London’s ancient guilds (livery companies). It’s right next to London Bridge, and he was attending an event there while on day release from prison. It has a lot of fishing related stuff on display - he and another man pulled two tusks from a mounting on a wall as the best available weapons.