Jewelry Marking Question

I have recently been watching pawn shop shows on the history channel and trutv" which got me interested in what I had that might be worth something. I dug into my safe and most of my jewelry have clear 10K-18K marking except for one pair of earrings that simply say AU. Did someone just stamp that just to indicate it is gold toned or is their a real gold indicator AU that I am unaware of? Thanks for any help.

( I don’t plan to sell anything to a pawn shop, this was for my personal interest only. )