Opinions on a vintage Longines

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This belonged to my mother's friend's husband's father (quite a mouthful) and looks like its going to be fully restored by him to pass on as a family heirloom. Wonder if I could get some details on the piece:

53f8371c.jpg
 
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There's probably a 23ZS inside, or maybe a 1x.68 of some sort (12.68 or 10.68) - either way it's a nice caliber from the 50's. A long shot would be a 22LS. It's already been redialed once so it won't hurt to do again if necessary. Pop off or screw off back?
 
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Don't have any more pics just yet, the dial was my biggest question, if its not original the discolouration would be worth correcting.