A big omission in many databases of economic statistics for the 20th century is the communist countries, which at one time included more than a third of the world’s people. Their governments were secretive. They did not publish certain statistics and they fabricated others.
Historical research is going back and filling some of the gaps, or replacing bad data with better data. Starting a decade ago, the Central Bank of Russia began issuing a series of print monographs about money and banking in the Soviet period. Michael Alexeev of Indiana University, an expert on the Soviet and post-Soviet economy, recently made me aware that the series is now available online. It is in Russian, but readers interested in the subject whose knowledge of Russian is quite poor, like mine, can use Google Translate to understand the gist of the papers. I will eventually incorporate some of the data into Historical Financial Statistics.
Another central bank that has done much to make available material from its communist period is the Bulgarian National Bank, though the material is likewise not in English.