Here's a couple items that I've come across lately that contains R. Crumb art but isn't exactly Ug-C that I'm not sure if they are included in the R. Crumb Checklist or it's Update in an issue of CounterMedia or the Crumb-Ology or it's Supplement.
Cover art to a 1970 book The Limits of Protest
Poor Richard's Adzine (fanzine)
I would think any Crumb art is going to be of interest to most underground collectors, at least the ones that like his work. Don D. has a few interesting items, including an issue of People Magazine with a Crumb profile and three illustrations that were specifically done for the issue.