In a message dated 2/4/2014 12:58:26 P.M. Eastern Standard Time,
jim.felder@GMAIL.COM writes:

I  recently acquired a brand new rubber bumper strip for the rear
chrome-style  bumper. Part number 251 807 421. From the rubber side, it is
like very  other one I have ever seen. From the backside, the holes for
mounting the  attachment buttons are not the usual slots that allow you to
snap in the  plastic buttons, but are double horizontal slots at each
attachment  location.

The two recesses for the bumper attachment bolts are in their  usual places.

Does anybody know what kind of mounting buttons this part  needs, and where
to get them?

It's a VW-AUDI part, if the imprint is  to be believed.

there were 2 different types of clips that vw offered for the rubber impact
 strips for the chrome bumpers. one was plastic (fits in round slots)...the
other  was metal (double horizontal slots).