Based on Early Mongol Flags at crwflags.com:

One of the charges is a crescent and the other looks like a simplified form of the tamga from the flag of Idel Ural. On different copies of the flag, the crescent has different size; it is often smaller than shown here, sometimes even reduced into a simple oblique stroke and conjoined with the other charge into a si[n]gle symmetrical object; the other charge also sometimes lacks the oblique part [2, 3]. It was obviously difficult to draw the charges always the same way. The cities with this flag which are easy to identify are [2, 7, 8]: Sarai, the capital (spelled Sarra) - there is also a depiction of the ruler, "Jani Beg Lord of Sarai" ("Jambech senyor de Sarra"); Tana, present-day Azov, Russia; and Urgench, Uzbekistan (spelled Organci, with a cedilla under the c; nowadays ruined). This flag is a variant of the flag of "Emperor of Sarai" ("Emperador de Sara") from "Libro del conoscimiento de los reinos" [7] and might be the one that had really existed, considering the similarity of its charges with those from the flag of Idel Ural.

[2] Enciclopedia universal ilustrada, vol. XXI, Espan~a Madrid: Espasa-Calpe S.A., 1968

[3] Istorija otkric'a i istraz<ivanja, vol. I: Poc<etak istraz<ivanja; Mladinska knjiga, Ljubljana, 1979; Original title: A History of Discovery and Exploration, vol. I: The Search Begins;(C) 1973 Aldus Books Limited, London

[7] Libro del Conoscimiento. Viajes medievales, vol. I Madrid: Fundacio'n Jose' Antonio de Castro, 2005 ISBN 84-96452-11-5 (complete edition) ISBN 84-96452-12-3 (vol. I) [e9s50]

[8] A[p]pendices. (Ibid.)

Tomislav Todorovic, 21 April 2007