Interesting article in German.Specialized literature is needed for these overprints.Impossible otherwise.Partial translation below
What about counterfeiting, how to buy these stamps? Wie sieht es mit Fälschungen aus, wie sollte man diese Marken erwerben?"As already mentioned, there are countless counterfeits, some of them are already without comparison material very easy to expose, as the overprint with other letters, another printing process was done, or the counterfeiter wanted to produce great rarities, such as double, inverted overprints, imprint on other stamps, mixed print, so once thin and once thick. All of these stamps carry false overprints, sometimes they are also offered as waste paper or misprints, but the stamp control of the PTT and the responsible officials at the IKW department worked well and Auberson's report clearly stated that the complete deliveries were flawless. Easy to unmask are hand stamp overprints, imprints with laser printers or inkjet printers. On the other hand, there are forged overprints which were made in the printer's pride shortly after the official print job, these unfortunately make up most of the fakes. These counterfeits are only with good comparison material and experience to expose, since the printing was done with the same machines, the same ink and the same block letters. The only difference is that the print forms have been reassembled. Therefore, each of the 25 types of imprints differs somewhat in the spacing of the letters and in the small particularities of each individual letter. In the book by Michael Peter and Ralph Soderberg these differences are explained in detail for the thin imprints."The situation is a bit different with the thick overprints. Again, these are overprints that were created at the end of the printing period with newly put together plates. They also differ from the real imprints by small deviations. But the printers went out of their way and printed in the evening for dealers and collectors with the real printing plates, on genuine stamps, with the correct printing ink, but not on behalf of the post office. I call them "Feierabenddrucke". So they are real, but not made by order of the post office. They are no different from the stamps that were delivered to the post office. This is probably the reason that stamps with the thick overprint type were more in circulation than the thin overprint type. Canceled stamps with a thick overprint are much rarer than mint stamps."