The Cummins service manual lists two damper styles. One is two piece rubber damper. The other is one piece fluid something or other. I have not seen a two piece rubber damper. I have only worked with the one piece. They list different speeds as well. The two piece is listed as 2,500 and below. The single piece is listed as 2,500 and above.
This doesn't sound like it jives with what Paychk found.
I also just found this;
Viscouis one piece, 2500 above
Rubber element two piece, 2500 below.
We are probably all wrong.