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Looking just at the Halcyon catalog, the Explorer knife sheath is stitched in such a way that the sheath sits at a near 50degree angle (from horizontal) on your waist belt. The knife goes in a little past the hilt, so I assume friction alone holds it in.
The H knife sits at 90 degrees and has an added top band that goes over the H knife handle. I think the top band might be elastic but I can't confirm this. The H knife goes in below the hilt, so I believe the top band is necessary to secure the knife.
Both sheaths are the same length, but the H sheath seems to have a shallower pocket for the blade.
The difference btw both knives is that the Explorer has serrations while the H knife does not. The H knife also has a molded handle where as the Explorer knife is naked titanium.
I personally would spend the money making my own DIR knife from a steak knife and spare webbing. You can make 2 sheaths and plenty of spare knifes for half the price of 1 Halcyon knife.
...What you're actually paying more for is the sheath - and the sheaths are very good.
...and by sheath, you mean little blue H?