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This lip usually signifies where to pry with a case knife.
If a lip is present on the back or the bezel, then it is a "snap-on" and not screwed on.
It is very common to find an expensive solid gold case with a low quality movement inside.
Even more common is finding an inexpensive gold-filled (gold plate) case with a fabulous high quality movement inside.
This is why it is so important to open the case to see what we have.
They aren't sharp enough to cut you, but are designed with a nice rounded edge that makes opening a case a breeze, and won't damage or scratch the case of the watch. A special adjustable case wrench is required to open this type of case.The companies would supply watches and cases to jewelers who would "custom fit" a watch for each customer. They would spend the extra money to have a solid gold case, but would "cut corners" on the movement to save money.Other customers wanted a reliable quality timepiece.This is a type of case where the back and bezel (which is the metal ring that holds the glass) are simply snapped in place like a Tupperware container. There will be a raised lip indicating where to place a tool for prying, but nothing else. Before putting one of these cases together again, carefully examine the mating surfaces, especially at any "corners." Usually there is a small locating pin, and corresponding hole, to assist in determining the correct rotational position for the back cover.
Many snap back cased, and wrist watch cases are impossible to open without using a DULL tool.The first step in learning more about your watch, or trying to repair your watch, is to get the darn thing open.