This has finally been resolved, in case anyone else has had similar troubles. First, a huge thanks must go to Astroman, who was kind enough to send along a complete K1 kit obtained at his local dealer. Much appreciated, and it says a lot about the people who frequent this forum.

At any rate, after bringing the kit (complete with receipts, bags and stickers clearly marked) to my local dealer, they did some digging and came to the conclusion that the solitary spring I had first received was mislabeled. They ordered another kit using the same PN and it is now in my possession.

Apparently, the part number can't be found on their computers, as it's part of an upgrade and isn't indexed (or something similar). They had the resident diesel expert available as consultant, and got me the part.

It seems that lots of other parts people are having the same problem locating this item (Tom @ mbz.org, another mentioned above), so perhaps it's MB's problem.

Hope this helps someone, and thanks for all the advice and assistance.