Terry: A DS was my first Lefever, and the gun that convinced me Syracuse Lefevers are the most graceful (and rarest) of the major American doubles. Others here know a ton more than I do, but since you're new to Lefevers and curious about them, here's a couple of facts.
The two boxlike X's stamped in the left-hand side of the water table are the factory's designation for their lightest frame; certainly appropriate for a 20-gauge (maybe all DS 20s would have used this frame ... I'm not sure about that), but definitely considered a bonus on 16s and a very rare find on a 12.
Guns like yours would have been built for, at most, about 15 years, between the turn of the century and the start of the First World War, when Lefever production ended forever.
I think almost all Lefevers you'll find with the rounded, open pistol grip are DS grades. A capped pistol grip is much more common on all other grades.
DS stands for "Durston Special." Durston was the name of someone affiliated with the Lefever company, but I can't remember who. All other grades were ranked and designated by letters of the alphabet, from A to H.
Finally, some advice. Be kind to that 100-year-old lady. Her barrels and stock will serve you well if you use the kind of loads that were available when the gun was built. Yes, your new gun was built for 2.5-in. shells and that's what you should use, but even more important is to feed it low recoil loads designed for old guns like yours. RST and others can help you with that.
It's a nice gun. Enjoy it, and welcome to the Lefever club. You are obvious a discriminating shotgunner! TT