Your question made me all the more curious, so I went out to the car and got the disk. The part # on it is 0001845725. For those of you who speak German, it has "Siebscheibenfilter-Einsatz" embossed on the top. (Does that loosely translate into "Makes the filter harder to install"?) It is 3-1/2" in diameter, and is indeed a "double leafed" affair with a centre hub- all made out of tough nylon plastic. And it does slide over the cannister bolt and sits on top of the filter cartridge. At first glance it looks like it could be a pulley out of a big tape recorder. Upon closer inspection, I noticed that the centre section has 8 spokes radiating out from the centre hole, and between each spoke there are two little oblong holes (oil journals?)- each hole leading to the outer top and bottom "leaves" of the disk. And much to my surprise, those leaves actually consist of an extremely fine plastic filter mesh! So this isn't just a disk to take up space- it's actually a filter- just as you call it! Now I'm listening!

This opens up a whole new line of questions- what is the correct way to clean this thing out? Does it ever get beyond cleaning and have to be replaced? Which actually plugs up first- this disk filter or the disposable cartridge? And finally the real question- for the sake of others because I'm not driving the car until I get this put back in after my proper cartridge finally gets here- what is the real danger of not having it in there? What if someone went to Jiffy Lube and the trained monkey decides not to put the disk in because he had the same problem getting everything to bolt up as I did? What if it gets thrown away without telling the owner? Is it just the W115 and earlier cars like mine that use this thing?

This could end up being a long thread!

Dave :p