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.
+ 1
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.