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.
+ 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.
Enjoy and take care of the watch that your grandfather gave you. You will not regret it later in life!