The FSM for the 617 shows the seal ring installed between the prechamber and the head. The collar should be screwed down directly onto the prechamber. I wouldn't think you would want to put another sealing ring between the collar and the prechamber. The sealing ring is narrower than either. Here is a link to the diagram I am looking at:
http://mb.braingears.com/123_DISK2/program/Engine/617/07_1-230.pdf
So if you want to replace those sealing rings you need to pull the prechamber. They also seem to come in two different sizes, 2 mm and 2.3 mm. The part site I looked at says 2 mm is standard.
I too have some bubbles at the base of the injectors, by the way.
EDIT: Just noticed you guys are talking about the 603 mostly. So what I said probably doesn't apply.