Parts question - Bilstein Struts
 

I'm in the process of replacing the inserts in a set of Bilstein struts.
When I disassembled the struts I found a black ring around the upper part of the old inserts, resting on a ridge about 3 inches down in the strut housing.
O.D of the ring = I.D. of the housing.
I.D. of ring = O.D. of the upper end of the insert.
The ring is about 8-10 mm in height, and is made of black plastic, with a small (2-3 mm diameter) metal coil around the circumference of one edge.
It looks like a dust seal or a device for centering the insert in the housing or ???.
Both rings are worn - and should be replaced. New rings were not included with the new inserts.
I called Bilstein - they said the rings are Porsche parts.
I've talked to several Porsche dealers, including Stoddards, and none had a lisiting for them or had ever heard of them.
Is anyone here familiar with these rings and know a source for them?
Thanks for your help.
Joe Frantz