It’s librarian codespeak for the National Union Catalog of Manuscript Collections (NUCMC). NUCMC is fifty years young this year-- you can read more about it here:
For years NUCMC was only available as a huge multi-volume book set-- now you can search it for free via WorldCat.org:
Hint: To do a quick & dirty search for NUCMC items relating to a particular state, enter these search terms:
missouri nucmc (substitute the name of whatever state you want to search).