Funny this came up...
This has had some light restoration work (pressed, re folded, cleaned lightly) but it's one of 3 examples of the 24c rate to Italy via Bremen-Hamburg Mail extant and by far the nicer of the two franked with a single 24c stamp:

Chip, this is the cover that we were discussing in Hartford.
As Chip illustrated, rarity is where I can certainly overlook condition. There's no reason to buy, say, a piggy 12c or 24c cover to England.
The significance of this cover is that it's one of a handful of very short lived Bremen-Hamburg mail rates that were in effect for most of 1867. Some of which are extremely rare or unique.