Hi Guys
Is the rear axle seal you are talking about?
If so don't let the lack of this seal stop you from moving forward on the truck. Reason I say this is one of my C60 trucks is missing this one but I don't know which truck. This seal has been sitting on the shelf, left over from over hauling one of the trucks. It has been on the shelf for just under 10 years, removed because the leather was ripped.
Now for the funny part neither trucks has leaked lube into the rear brakes. Now on the C60S with the radio box I'm not surprised because it gets lots of miles but only on the road, and little to no cross slope. Now the C60L has had lots of pretty sever cross slope operation and no sign of any lube leaking.
Cheers Phil
PS If you have ever wondered why many of my parts photos have a map as a back ground. I use them as clean work surfaces because I've got cases of out of date maps.