I looked through the section of Hannover in the Michel Catalogue, read the introduction and several others and not a mention of this. I have quite a lot of Hannover but not another one of these. Looking at this again I should have noted the posthorn since I have the others with the posthorn on white background. Where did you see this, I mean in which Catalogue?.
The word "Bestellgeld" means "delivery charge", much the same as Zustellgebühr, so we basically have "free delivery".
From what I can make out from the Michel postal stationery catalogue, it's a cut-out from a Hannover town post envelope from 1858 or, perhaps more likely, from an 1863 reprint.
There was a similar design with a rearing horse from 1861.
