From Stamp Forgeries:
16-23. Handstamp overprinted revenue stamps, 1926. Forged overprints by Chtabinsky, Louschnikoff, Golding, Chun, and others and the Alton Bulak forged overprints and forgeries by Sekula and Refky of both the basic revenue stamp and overprint: Alfred Orth, "The 1926 Overprints Of Mongolia: Only One Type Of Overprint", Journal of Chinese Philately, Vol. 26 (February 1979): 57-63, continued at (April 1979): 82-87. W.H. Adgey-Edgar, "The 1925 Overprinted Fiscals Of Mongolia", Journal of Chinese Philately, Vol. 16 (April 1969): 73- 76. Chtabinsky forged overprints and other forgeries. Gunner K.-E. Hellstrom, "Forgeries of the 1926 Provisional Issue of Outer Mongolia", The Philatelist, Vol. 23 (January 1957): 87- 91, continued at Vol. 24 (February 1958): 134. Measurements of the Urga counterfeit overprint: E.M. Tolman letter to Editor, Mercury Stamp Journal, Vol. 4, Whole No. 38 (June 1956): 47.
48-54. 1931 Provisional issue. Forged overprint on genuine stamps and complete forgeries: Dr. Alfred Orth, "The 1931 Provisional Issue of Stamps for Ulan Bator", Journal of Chinese Philately, Vol. 36 (August 1989): 122-24.
59-61. Provisional issue, 1931. Forged surcharge. A. Orth, "Forgeries Of The Mongolia May 1931 Issue", Journal of Chinese Philately, Vol. 25 (December 1977): 21.
Forged and bogus cancellations: Wolfgang Hellrigl, "Forged and Bogus Postal Markings of Mongolia", Fakes Forgeries Experts journal, No. 12 (May 2009): 55-70.
Someday I am going to source those articles out. I think that you almost have to assume that unless it's got a certificate, it's faked. For example, matthijs_philatelie on eBay
is selling faked multiples every week. He is also selling multiples of the 2 1958 issues for stupid money that are not particularly rare--it's obvious he has a sheet or two of them and it's a cash cow for him, but if people are willing to buy a block of stamps for $150 it's up to them. I figure at some point the price will plummet and I'll snag a copy.