Operator's, Organizational, Direct Support, and General Support Maintenance Manual: Ohmmeter ZM-21/U, (NSN 5950-00-645-2191), Ohmmeter ZM-21A/U, (NSN 6625-00-643-1030), and Ohmmeter ZM-21B/U, (NSN 6625-00-581-2466).
Jan 1977 ยท Headquarters, Department of the Army