I’ve sure tried my hardest to figure out what exactly is the story behind the UM-series Michigan plates. Certain county code lists list UM as being a Washtenaw County code (presumably because of the University of Michigan’s being located in that county), and others don’t list it at all. I’ve only been able to confirm this coding anomaly in 1965, 1967, and 1968 — I’ve never seen a 1966 or a 1969, nor have I heard of this being a phenomenon in any other years.
Anyhow, here’s the little set I’ve been able to assemble. If you have any information regarding these or if you have photos or examples of 1966/1969 UM series, please do get in touch with me.