Sorry for the delay . . . .
I borrowed these courtesy of my friendly local Volvo parts man. They are plastic (probably plated with "jenuwyne krome")--I guess they're similar to what markb mentioned in an earlier post.
They look pretty good, but will take some messing with to make them fit perfectly. The lower picture shows the little wedges inside the cap that make it fit tighter as it is pressed on. These wedges are a little too thick for the 914 lug, and they keep the cap from completely seating (about 3/16" away, as shown in upper picture). Also, not sure how the caps will hold up over the long term (flaking. cracking, etc.).
As you can see, my lug bolts look pretty crappy--I may just spring for these and see what happens . . . worst comes to worst, I can unload them on evilbay
Volvo PN is 9139853, cost is around $20 for 20 caps. Not sure who makes them for Volvo (made in Taiwan), but picture on back of package shows them in black too.
As grandpa used to say, "ya pays yer money and ya takes yer choice . . . ."