Hi Martin
As others have said, knowing what a particular case reference should look like is just about experience.
Study as many examples as you can and keep asking questions - members are happy to help.
The references 14381 and 14393 (no date and date versions of the same case) sit bang on your birth year, so perhaps you could focus on these references.
(There are others but they are either at the end or beginning of their runs and so less common to find)
Regarding the crystal, an original would have a tiny Omega symbol in the centre above the fulcrum point of the hands.
Whilst after market crystals will fit the watch, they often have an incorrectly sized retaining ring (evidenced by both of the OP’s examples) which also doesn’t provide the waterproofing it should.
Hope that helps.
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