I have never actually seen one, however in the early Daguerrian years of photography a right angle prism was commonly used to "correct" normal reversal of images. So, when you see a daguerreotype with writing running in the correct direction, chances are the photographer used a prism.
I have used the Kodak Prism, the second eBay auction you reference. As long as the mirror hasn't tarnished, these are remarkably high quality and don't actually reduce exposure by very much. They do cut down on contrast, since they add a number of uncoated glass surfaces.