Dangerous forgeries of Swiss airmail stamp.
This is also the case with stamps 24 and 24a of the Zumstein. The 24 a is the first edition of an overprint stamp with a total of only 20 000 copies. Afterwards, there was an additional edition (in DARK red) of 320 000 copies.
The first, very rare edition was printed in a LIGHT red colour. Forgers have made dangerous forgeries of the original stamps, the “90 rappen” (Zumstein no. 19), whilst others wanted to delude a light red edition with chemical discolortion.
But there are also other examples, like e.g. false copies on letters. A forger bought some of the unused 24a copies and halved them vertically. Then he soaked a real “90 rappen” from letters, stuck the left half back on the postal item and added the imprinted half to it.
Using two halves of two different stamps, he thus imitated a real copy with a real light red print. A very rare bit!
The added piece usually hasn’t got any stamp fragment, but even stamped, caution is required! It is advisable to ask for a certificate!