One man has the TS slip, the other the Rubaiyat it was torn from. They meet and confirm their identities by matching both torn objects before one departs with the book and the information hidden within one of its pages.
That’s the theory.
Here are the precedents.
(1) The Identification Process.
After wrapping up the details of the Fuchs meeting, the handler told (Harry) Gold something surprising: the Soviets had a second mole inside Los Alamos. This mole would be in Albuquerque, not far from Santa Fe, while Gold was visiting, so Gold needed to make a side trip there to pick up additional papers from a man named Greenglass.
The handler then gave Gold half a Jell-O box top, which had been cut into a jigsaw shape. He passed the puzzle piece over to Gold. You’ll know it’s Greenglass, he said, because he’ll have the other half.
Harry Gold: Spy in the Lab – Sam Kean April 25, 2019. https://www.sciencehistory.org/distillations/harry-gold-spy-in-the-lab
Somerton Beach, 1948. The agent and his handler were to recognise each other by one producing a slip of paper torn from a book and the other the book it was torn from.
(2) Following The Signs.
Foxstone Park, 1910 Creek Crossing Road, Vienna, Virginia. FBI Agent Robert Hanssen, acknowledged as one of the most damaging spies to the U.S., signaled his Russian handler by placing a piece of white tape on the entrance sign to Foxstone Park. The tape confirmed that Hanssen had placed a package of secrets in a dead drop, code named ELLIS, located under a footbridge inside the park.
Somerton Beach, 1948. An agent signaled his handler that there was information to be found hidden in a specific place within a book by using a pre-arranged series of letters included in an otherwise meaningless code written on the back of the book