A reconstruction of the flag of the Mongol United Autonomous Government described by The Airpost Journal on a 1937 publication. It is described as "a flag of blue with a square of horizontal red, yellow, white and black stripes".
Based on the currently more popular SVG for this flag, with the blue stripe in the canton replaced with black, as the Airpost Journal mentions a black stripe instead of blue.
https://web.archive.org/web/20170107010250/http://www.americanairmailsociety.org/APJbackissues/Volume%208/APJ,%20Issue%2085,%20Vol%20ViII,%20No%208,%20Mayl%201937f.pdf
Flag_of_Mongol_United_Autonomous_Government_(1937-1939).svg: Marce79
derivative work: Infrish