The next time you flip through your collection of unidentified photographs, turn them over. The hacks of the pictures may be more interesting and informative than the images themselves. And if you know what to look for, they could hold the keys to identifying those mystery faces. So grab your magnifying glass and start examining your pictures for these telling details:

Revenue stamps

Between Aug. 1, 1864, and Aug. 1, 1866, the US government levied a tax on photos. Photographers had to affix a stamp to the backs of their images, and hand-cancel each stamp with their names or initials and the dates of sale. That date can help you compile a list of ancestors who lived in the right place and time. Narrow that pool by matching your ancestors' sexes and ages with those of the people depicted.