Tinker, Tailor, Soldier, Spy discussion

"I'm curious: towards the end of the novel, when Smiley is questioning former agent Jerry Westerby, Westerby makes a specific point of mentioning a curious Christmas card which he received from Toby Esterhaus. No message, just 'Happy Christmas, Toby', and a picture of Leadenhall Street. The fact that he made special mention of the curious choice of card made me certain that this was a clue, but in the end it was never followed up. What do other readers think? Was it simply window dressing? Or a red herring? I had an idea that Toby was hinting that he knew about the Tinker code, to see whether Jerry reciprocated, where the 'Lead' in Leadenhall Street referred to 'soldier' (lead soldiers). Nothing came of it though, hence my question.

Paul wrote: ""I'm curious: towards the end of the novel, when Smiley is questioning former agent Jerry Westerby, Westerby makes a specific point of mentioning a curious Christmas card which he received from Tob..."

This isn't a definitive answer, but Leadenhall Street is best known as the site of Lloyds of London, which insures insurers; I think Toby is presumably telling him he's keeping in touch as some kind of insurance policy.