The plates would have been for Niepce’s process, the physautotype, not for the daguerreotype. It was very different from the Daguerreotype , he used iodine to blacken exposed portions of silver on a plate, post exposure and partial development. Daguerre’s approach was different of course, using Iodine as a sensitising agent, his discovery and use of latent image development and of course the stumbling block that most would-be inventors of photography struggled with, he halted the action of the light sensitive silver salts post development by fixing the plate in a saturated salt solution. His initial version of the process did have very long exposures but they were much better the physautotype’s which were several hours. The plates mentioned in the letter above probably bore photomechanical images rather than images from nature. Niepce like Talbot was more concerned with a process that would allow for the mass reproduction of images. Daguerre’s aspiration was less mundane and sort the perfection he could see in a mirror.