In the years since the issuance of the Krakow issue, a number of individuals have acquired the necessary reference materials and have sufficiently studied the overprints so as to develop an ability to render an opinion as to the authenticity of an overprint. Usually they would place some distinctive mark on the back of the stamp that they had determined to be a genuine overprint. I have displayed below a few of these marks, and I have included only those for whom I have genuine marks. I do not have a list of those individuals that would be considered to be recognized experts of this issue, and these reproduced marks are just examples of the type of marks to be found.

However, I must point out that these marks can be and have been forged, so the presence of a mark (or marks) on the back of one of these stamps is not necessarily a guarantee of authenticity. The measurement of the height of the overprint on the stamp shown below proves the overprint to be a Mueller forgery, yet note the marks on the back of the stamp.

In the purchase of one of the more expensive of these overprinted stamps, the only safe course is to purchase one with a certificate of authenticity which implies a money back guarantee if the overprint is subsequently proved to be a forgery.

I will again repeat the opinion of one of the early experts, that in his opinion in 1927, over 60 percent of the common and over 90 percent of the rarer overprints are forgeries. Beware of bargains. The odds are very great that a stamp which have a catalog value in excess of a hundred dollars and has no certificate will be a forgery, worth less than the reproductions that I create.