The acorn was a method of communication used by Oakley Street. It was a nut-shaped container carved out of two pieces of tight-grained wood. It had two parts - the cup, which had overlapping scales and was stained lightly with green and the nut, which was polished and waxed brown.
The acorn carried messages between Oakley Street members and was left at drop points to be collected. When opening the nut, the cup had to be turned in a way that appeared to be doing it up tighter. It took a dozen turns to open. The messages inside were written on thin India paper.
An agent responsible for picking up and replacing the acorn was referred to as an 'insulator' and the collection points were called 'left-luggage boxes'. The insulator and acorn recipient did not know each other to make questioning more difficult.
At sometime during 1986, the nut was being carried by Robert Luckhurst. However, he lost the nut before leaving it at the drop point, where it was meant to be collected by Hannah Relf. Luckhurst and Relf used nine different left-luggage boxes, the order chosen using a code rather than strict rotation. This approach allowed four or five officers to regularly consult Hannah without fear of a noticeable pattern being established. She was aware that something was wrong when the acorn did not appear at a certain point in the University Parks.
Malcolm Polstead found the acorn, having witnessed Luckhurst being taken by the Consistorial Court of Discipline, and later returned it to Relf. Before handing over the message inside, he tested whether she knew that the acorn unscrewed clockwise. Whilst Oakley Street organised a new insulator to take over from Luckhurst, Relf found a half-dozen new left-luggage boxes to prepare for resumed communication. The first was a space under a thick root in the Botanic Garden, inside one of the hothouses.