It is a simple rectangle of metal with three squared off prongs at one end.
The middle prong fits where the banana plug goes and the two outer prongs fit the two small cutouts on the terminal.
Originally they were supplied with the speaker but were quite soft and didn’t last too long. A local retailer may be able to find one in a bottom drawer.
Plan B is simply take a pair of needle-nosed pliers, insert into the two small terminal cutouts, hold carefully and twist anti-clockwise to undo.
Edited - Pete at Cymbiosis advertises them
https://www.cymbiosis.com/product/linn-key-for-speaker-socket-collar/