This work categorizes Japanese Sign Language (JSL) toponyms, or place names, and examines factors that potentially affect their structure. Exonyms, influenced by the source Japanese name, and endonyms, independent JSL names, contrast structurally in that exonyms tend to emerge as compounds while endonyms conform more closely to canonical monomorphemic JSL lexemes. Based on the contrast, one would expect the structurally more favorable, less marked, endonymic outputs to emerge as the norm; however, structurally inefficient exonymic forms do so. This study analyzes a collection of over 900 names from the 2009 Japan National Federation of the Deaf (JFD) National Toponym Sign Language Map to determine structural influences on JSL toponym outputs. A spreadsheet tracked morpheme counts and relationships between source name characters and their outputs to find categorical distributions. This study finds that JSL toponyms tend to disproportionally borrow Japanese morphemes associated with JSL character signs that map to the source characters. A character sign is a sign isomorphic in some respect to a Chinese-origin orthography. Favored indexation to such source morphemes demonstrates that source name familiarity can support the persistent spread of exonymic toponyms.