After I purchased the latest Standard Catalog of World Paper Money I was forced to update the list of countries at RealBanknotes.com because there were a few additions to be made. Tannu-Tuva was one of them.
Tannu Tuva was an autonomous part of the former Union of Soviet Socialist Republics located in Russia and bordering Mongolia. The country declared its independence in 1921 and was re-absorbed into the USSR in 1944.
During its existence their paper money consisted of a Russan issue with a Tannu-Tuvian overprint, until 1935 when it printed its own currency, the Aksha. Now-a-days it is difficult to find any Aksha banknotes, though the overprinted Russian notes can be found on eBay and auctions seem to end with prices starting at around the $12 mark.
Here is an example of one of the early, overprint issues (5 Lan on 5 Rubles):