Ref 5557 isn't in Richter's book but the number, and shape of the watch, is consistent with the numbers that are in the book. There were a lot of variations of these vintage pieces and not all of them were well documented. Because of their size the ladies watches are often without serial numbers. Just not enough space in the watch to include them. The 55XX series of Ladies reference numbers does include a lot of 18k and plated examples so having a pic of the 18k stamping would be comforting. No reason to think it's not original but there's a lot of value difference between plated and 18k.