I am in possession of an envelope of 7 very small images. They are all about 1-1/8” by ¾” and are all head shot portraits of people in dress from the turn of the century. They seem to be very detailed considering the size of the image. I believe they are printed on metal, perhaps tin; however, the color on the back of the picture is dark brown and looks like it is coated in resin. Can someone tell me what these are and why they were made?