Found this rummaging through 102 cards at a show years ago. I think the horseshoe cancels (I've also heard them referred to as "sausage" cancels) were used from around 1864 to 1880 -- I'm sure someone on SOR knows these details better than me! This is the only example I've ever seen on a DWI stamp (not saying there aren't others, though).

The 7 Cent issue paid the first class letter rate from the islands to Germany and Denmark onboard German ships. This example is the from the 1st printing, having the "normal" frame.