For those of you who came to this page from the general "Display Dummies" page, the text below will be like deja vu all over again. This information is provided for those of you who came in from another direction, so if you've already read it you might want to skip down to the links below (unless you REALLY liked it the first time).
Many camera manufacturers sold dummy cameras to camera stores for display purposes. They were quite a bit less expensive for the dealer than a working model, and were of use when they were displayed in cases or windows were they would not be removed to demo the camera. They were also used in high risk areas, where if they were stolen the store would be out less money, and the thief would have something of no value to them. The thought was that might help deter thefts.
Generally dummies look complete from the outside, and exhibit differing amounts of completeness inside. Sometimes they have functioning subassemblies inside, such as pressure plates, sometimes not. Often the lenses are only the front element, sometimes blacked out on the inside with paint or a metal plate. Again, sometimes not. It's possible in some cases that subassemblies or lenses which did not pass inspection may have been used. There is no guarantee that all of the dummies of a particular model will have identical internal construction. I would not have been important to the manufacturer.
Many of the dummies have some markings on them to indicate a dummy camera. Leitz marked their dummies with an "a" after the serial number, Rollei cameras have a "0" in front of the serial number. Nikon engraved the word "DUMMY" on the backs of Nikon F cameras and lenses, yet did not mark rangefinder cameras. We had a Voigtlander Vitessa marked on the bottom "DUMMY", "ATTRAPPE" and "FACTICE". Many are not marked in any way at all.
Although dummies shouldn't show any wear as they could not be used, and would have spent their time in a display case, many show up that have been abused. Once their useful lives were over, they were often discarded and used as a source of what parts they did have to replace knobs, levers and anything else that could be salvaged for store use on working cameras. As a result it is common to find them incomplete.
Although they were less expensive than working models, they still cost the dealer a fair amount of money, and generally only the largest camera stores could justify the expense as unlike a demo camera, they couldn't really be sold. Because of the low sales and the fact that most were stripped then discarded when the model was discontinued, they are very hard to find today, and depending upon the model, can command as much or more as a working model.