Steve CT is correct. There were originally four barrel shroud configuration options:
Standard (solid rib, underlug just past the ejection rod)
Vented (vented rib, lug as above)
Heavy (solid rib, full underlug)
Vented Heavy (vented rib, full underlug)
I purchased my first one in 6" with the Vented shroud, as it was primarily intended for open carry while working on a small cattle ranch in SW Missouri and finding ready-made holsters for full-lugged revolvers was a lot more difficult than now, being pretty much limited to those intended for the Colt Pythons.
While a bit heavier than a 4" model, I knew that I could shoot with a 6" barrel a good deal more accurately especially out at 50 yds. and over. Coyotes and other varmints were a constant problem, and I could collect a $15 bounty from the State for each pair of coyote ears I turned in.
One of the major reasons I chose the Dan Wesson then was that I could only afford to own a very few, purpose-driven firearms. The ability to easily and relatively cheaply modify my sidearm for different task sets by simply getting another barrel and/or shroud was a huge "plus", as it required only a fraction of what it would cost to buy a seperate revolver for each. I've never regretted my choice, as it has worked out to be just as practical as I'd hoped it would.