This is the first time I've seen the "DS" ring.
Did anyone else notice it in previous episodes?

Yes, they've shown it a number of times, but the only other time they called any attention to it was in the pilot, when Kate was skeptical of Charlie's claim to be in Driveshaft - he flashed the ring and said "Second tour of Finland".

Thanks! I'll have to go back and watch it again. (One more thing to put on my "Must do" list.)

I've seen it a few times myself before.

They showed it a lot in season one.

Doesn't Dominic mention it on the 'Fire+Water' documentary? Pretty sure he said that he always makes sure to wear it.

Also when Charliewas detoxing on the plane, he was tapping it against the metal part of the seat's arm.

You also get a replica of the ring with the McFarlane Series 1 Charlie figure.

Is the OP serious?

Charlie has had it on in every scene (except certain flashbacks) since the first episode. He's flashed it several times and and it's been in camera shot on dozens of occasions. He showed it to Kate, he was taps it on things all the time. In fact, because of Charlie writing the words "FATE" and "LATE" on his fingers, we've probably had more closeups of HIS hands than anybody else's.