It’s kind of like a Letterboxer Trading Card (LTC), except that that card is 1"x1" square.

The line on the base plate of a compass that points to the degree that should be read.

When you are confused or not sure that you heard a statement correctly, you have ink in your ears. For example, you might say, “I must have ink in my ears—I thought you just said that you went to a brothel after finding my letterbox!” The first known use of this term was by wassamatta_u. Not a big shock there.