It is most likely 30L caliber indeed. I'd also say it's a 60s watch... Maybe mid to late 60s ?
The case back is a snap off which means you could take it off with something very thin and hard such as a knife but I advise care if you really want to do this. You should also look for a small specific place where a tool's blade can get inside to snap it off.
Inside you'll find the movement's serial number which can tell us what period was the watch made.
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