I always thought these rules were essentially a stylistic sound grab, and really not very informative of how distrustful parties rendezvous in adverse circumstances. Really? They say very little. I think spies use one time pads and dead letter drops and avoid contact.
The implicit artifice of Le Carrés yellow chalk and thumbtacks and poplars and meeting at 7 or 17 past tge hour.. they're not that different to "son of the charm" and magic tricks with coins and rhythmic paused speech in Kipling's "Kim"
They can't be very important or Le Carré would have been warned off.
