The original zytel handled Delicas and Enduras (with the plastic clip) were marked "G-2" steel, later ones were marked "GIN-1" (though I think I read somewhere that these are two names for the same steel).
In 1998, they revised the Delica/Endura line with the (what else?) "Delica98" and "Endura98"; these feature the texture on both sides, a reversible metal clip, and ATS-55 steel. These are the current production zytel handled Delicas and Enduras.
Around the same time, they introduced the "Delica II" and "Endura II", which are the stainless steel handled versions of the knives. These have blades marked "AUS-6".
I don't have a steel-handled Dragonfly, and my first generation zytel Dragonfly isn't ready to hand, but I'd venture a guess that they went through the same transformations (except of course they didn't do a metal-clipped "Dragonfly 98").
By the way, the original Dragonfly had a more curved spine, a less pronounced hump, and was hollow ground, rather than a full flat grind.
As ptn said, Spyderco is constantly tweaking and revising their designs, which is one of the things I really like about them...
I know, more than you really wanted to know,
-- Carl