Pocket watches are 99% with the back snap-on/sanap-off opening. Never came across the screwback. Get the knife with the solid blade, insert the blade in the groove between the case and the caseback and push it in quite hard using lever action (pry up). At the same time make sure the blade do not slip into the movement, as it will damage your watch.
If there is no groove between the case and the caseback, then watch must be opened by removing bezel (metal ring holding glass in place). Open the same way as for the caseback, just be VERY carefull, as any slip of the blade will leave the marks on dial.
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