"When he was a little boy, back in Leningrad"

This is inaccurate.

It is earlier stated that Tchitcherine was born in 1905. Petrograd, now known as St. Petersburg, was only renamed Leningrad in 1924. Tchitcherine would have been 19 by this time. Whether this is an error on Pynchon's part, or whether Tchitcherine is simply applying the city's new name retroactively, is unclear.