These smaller 3-part multis were once common on the Dakota Public Service / Northwestern Public Service system, so I wonder if it was one of the 'standard' types specified across the old Middle West Utilities system. In the decades since, these have rapidly given way to 45kV-class 2-part multiparts (and posts), and are now rare stragglers across Northwestern Energy's South Dakota system and Montana-Dakota Utilities' Dakota Heartland district.