[WorldTime643]

Amazing to see that it fired up after so long!
That's a beauty with the 108 module, I have the same one on my W-250.
You might find it quite a tight fit to get a NATO strap on it though, might be better to consider a 2-piece alternative. You should be able to find an 18mm Casio resin strap that will fit, but it might not be a direct replacement.

 

[WorldTime643]

A Casio or some type of rubber strap is my first choice, as I would like to keep as stock looking as possible. I measured the lug spacing, and it looks like its a 19mm. I'm actually getting a bit curious about this particular model and early 80s casio watches. Any info on this watch would be much appreciated. Is it a collectible piece compared to the marlin models? Is the F81 much more rare than the F91? Was the G-shock available during the early 80s?

G-Shocks appeared in '83. I don't really know the relative rarity of the models you mention. To be honest only mint examples with original bands are likely to be collectable but certainly anything with a 108 is fairly unusual and you might even be able to pick up a replacement crystal for your watch.