Bruh

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Ironically the strap may actually have more value.

You may want to post it on vintage Japanese watch forums or dive watch forums with a Japanese section. I think that was an OEM Casio strap that might not be easy to come by today.

Casio 549 with a W-71 movement.
Edited:
 
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I'm actually only 15 so could you plz recommend a form I could post to
Enjoy the watch as a gift from your grandfather.
 
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Enjoy the watch for what it is - a gift from your grandfather - as such, value is irrelevant.
However, please keep up your interest in watches - it's great fun.
 
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Enjoy and take care of the watch that your grandfather gave you. You will not regret it later in life!