[CWizard]

Just to close this out, the wear marks were caused by excessive clearance between the rocker stud and the hole in the rocker arm trunnion. This, in turn, was caused by the trunnion holes being too large. The holes should have been .438" in diameter, but were .003-.006" larger than that. This has been confirmed by Comp Cams (rockers are their Pro-Magnums), and they are rebuilding the rockers with new trunnions under the warranty. Their warranty guy said he had never seen this problem before.
Although I didn't say this in the original post (because I hadn't at that time examined all the worn studs), some of the wear was severe enough that you could easily feel a ridge on the stud with your finger. The rockers were really digging into the studs.
Thanks for the responses above.