"AU" stands for "almost uncirculated." It means that a minor abrasion has occurred to the highest points on the coin. On currency it means the slightest imperfection to the surface. The pressure of persons handling it have caused indentations. It is not absolutely flat. Oh, and the description "funny back" is promotional talk for "If you haven't seen one it is "funny" to you."
Sounds like it might be better to buy already graded note if you are looking for a high quality note instead of taking a chance on a great picture of an ungraded note.
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