(German language) “1897/1904, ungebrauchte und gestempelte Sammlung mit besseren und kpl. Ausgaben, dabei u.a. Mi.-Nr. 23 ungebraucht und Mi.-Nr. 32-44 gestempelt, meist in guter Erhaltung, sauber auf zwei alten Vordruckblättern.”
For complete lot see the Heinrich Köhler website.
The two photos from the lot (copyright Heinrich Köhler):
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The stamp on the page annotated in pencil “12” is a forgery.
How do you know? By which I do not mean to imply you are wrong (I know you are on the official Prüferliste from the BPP), but what I mean is: what gives the forgery away?
By the way, if you check the link: lot 3932 was sold for 650 EUR (starting price 400 EUR), and the (German) description says nothing about the possibility their might be a forgery…
To answer the question how can one tell it is a forgery, a couple of things give it away; firstly the colour of the base stamp is wrong, but more importantly the Hangul aspect of the overprint is completely wrong. On this forgery (as on many forgeries) it reads 원 “won” or “one”, but the genuine overprint should read 일 “il” as in the number one in Korean.