Stamps on:

     Left corner, upside down means "I love you"

     Left corner, crosswise means "I love another"

     Left corner, straight up and down means "Wish to be rid of your correspondence"

     Bottom of right corner, crosswise means "No"

     Bottom of right corner, upsided down means "Yes"

     Left lower corner means "Do you love me?"

     Left lower corner, upside down means "I am displeased"

     Left lower corner, crosswise means "I wish to have your acquantance"

     Right corner, up and down means "Business correspondence"

     Left side, in the center means "Accept me as a lover"

     Left side, upside down means "I am engaged"

     Left side, in the center, crosswise means "Who cares?"

Source: "Fact, Fancy and Fable" by Henry Frederic Reddall