I'm in the midst of a project to study the ca. 2.3 million MARC records for archival materials that are in OCLC Research's ArchiveGrid resource. There are currently nearly 700,000 records that have one or more 856 links to digital objects. About 50% are visual materials,  40% are "mixed materials," and the rest are spread across textual manuscripts and scores and maps . 

Sliced another way, the 700,000 includes more than 300,000 imported from CONTENTdm sites (via OCLC digital collections gateway—these also go into WorldCat); of these, half are for visual and the other half for "mixed". Lots are linked to finding aids, of course. Much of the visual stuff is linked to images of the objects themselves.The nature of the links and resulting displays vary in a number of ways. (It's deeply satisfying to my inner geek to be able to get at all this data!).

Ed, if there's something in particular you're looking for, let me know, and I'll see what I can dig out.

Best to all, Jackie

ArchiveGrid  http://beta.worldcat.org/archivegrid/
Digital collections gateway  http://www.oclc.org/digital-gateway.en.html

Jackie Dooley
Program Officer, OCLC Research
Past President (2012-2013)
Society of American Archivists