Yup, rubber stamps! Mark is right there in the Grand Tradition. You also need a nice pad on the dry side.
Here's another one of some note. You needed double weight paper to stand up to abuse like this.
A silver print landscaper I know has a stamp with fill-in-the-blank spaces for edition number, image name, etc.
edit...and another one! Old photos with this kind of stuff are worth a lot more than prints without.