Back in 1994, Casio came out with the DW-6400. It was a different way to go about the G-shocks in a few different ways:
1. First G-Shock that didn't say "G-Shock" anywhere.
2. First to use under-wings (because the watch was so large!)
3. And the most obvious: it was the first with a metal exo-skeleton! Ultimately it is this why it is soooo famous.. . .
This watch has been written about before, of course by Sjors over on 50gs and by Casiophile in of all places, ebay!
This watch gave G's a whole new way: brutal, tough, intimidating. It is said this watch was popular with construction and other forms of tough work (in Japan), one of the reasons they say not many are left. Well, that's one reason. Another would be resin rot, you can see threads of the large resin block (under the metal) crumbling as far back as 2006. In any case, a cool watch that changed a lot of things that is a lot of fun.
I mean, just one look at it and all you can think is: "Cool!". That metal protector gives it such a unique message on the wrist. Don't be fooled about it's size though. As you can see it is smaller than today's XL's (GA/GD 100/110/120/150/200) but back in the day this was HUGE! They even made up the under wings to help such a large watch fit people (later to be used on the G7900's, etc). If you don't already know, these wings do help those with round-topped wrists. If you have flat-topped wrists (like me) then removing them will be more comfortable.
It may seem like a small thing now, but being able to make the watch without the G-Shock branding was seen as a big step by Casio. It had said a lot to how far Casio had brought the branding where this was even possible.
"Brutal/tough/intimidating" with the metal exo-skeleton to boot. You can see why it was nicknamed "Gundam" so easily after the Japanese cartoon about humongous robotized exo-skeletons. Not the end-all for this topic, but you can read more about Mobile Suit Gundam.
You will find some more info about the 6400's mixed in with the rest of the family, I am merely trying to separate for specific information on models here. Check the others out as well (DW-8100 and DW-8300 both coming).