Many times there are Tibet sheets offered which contains 24 stamps in 2 blocks of 12 stamps. Please note that there were no Tibet stamps printed in sheets of 24 and such sheets are private reprints done by the late Tibet collector and examiner Kurt H. Dahnke which were added to his famous Tibet handbook in German language.
It must be said that the sheets that were coming with his handbook were marked. The back of the sheets was painted in white color and the bottom 12 stamps had a small black round security marking in dark blue/black color.

WITH SECURITY MARKING

After his death, a large number of sheets of 24 stamps have surfaced which were NOT marked as above described and which quickly came into the hands of unscrupulous auction houses, offering them as genuine stamps.
WITHOUT SECURITY MARKING

Fortunately, I came across the original printing plates and purchased them for safekeeping.

Here are images of the printing plate and complete printing sheets.

Large size images (685 KB) of the printing plate and printing sheets can be downloaded from my server at :
http://fuchs-online.com/tibet/fake/tibet1.jpg
http://fuchs-online.com/tibet/fake/tibet2.jpg
http://fuchs-online.com/tibet/fake/tibet2.jpg