Stamps with ‘HAN’ numbers are Hausauftragsnummer, or were job orders (printing contract numbers) submitted to a printing house and numbered sequentially. This job order number appears in the selvage of stamps. I’m taking this information directly from Michel and PhilaSeiten.de (a really good philatelic translation website). Don’t have much more information on printing houses at the moment, but it’s a start.
Aus einem Amerikanischen Briefmarkenforum (14.09.08)
mfG
Nigel