Bonk (pronounced Byonk), with its subterranean twin city Schmaltzburg, is perhaps the most important city in Überwald. The area is famous for its fat geysers.
There has only been one stamp issue, which comprised three values. There was a very handsome 1 Bz stamp and a pair of 100 Bz and 120 Bz value. This seems odd, having no intermediate values, but there have been 2 and 5 Bz values from Muning, and 10, 30 and 60 Bz values for Überwald. So perhaps whichever place the stamps say is the issuing authority they are valid for postage across Überwald,

The 1 Bz stamp shows a werewolf silhouetted against a fat geyser. The lunar effect on werewolves is reflected  in the 28 daily phases of the moon along the stamp’s sides. The stamps were printed in sheet of 24, with no sports. Instead, there were seven transformations, showing the werewolf of the common stamp changing back to human form in steps. These are shown on a separate page. There appears to have been reprints of the stamp, evidenced by shade variations that can be found.
Originally there was an intention to have a 3 Bz stamp, a sister to this one, in brown, but this was abandoned. However it can be seen on the Artist’s Proof sheet for this issue.
The higher values feature the double-headed bat of Überwald, and are printed in the colours of blood - arterial for the 100 Bz stamp and venous for the 120 Bz. These both seem to have had a reprint resulting in shade variations. Continuing the blood colour theme, there is a paler anaemic 100 Bz and a darker polycythaemic 120 Bz - there may be more! They were printed in sheets of 40 stamps with one hard to detect sport on each - look at the twirly bits.

Three shade variations

The shade variations of both stamps
I like to think of these as arterial and venous blood with the 100B pale being anaemic, and the darker 120B as polycythaemic.