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1917–1919
The state took the road in 1917 and numbered it first among its highways. The line became Main Market Highway 1 in 1917 — the act was signed into law by the Governor on 7 March 1917, and the signs were installed on 1 June of that year. A state-highway history puts the arrangement plainly: "At the Michigan state line, original state road (OSR) 1 started along what was the Dixie Highway. At South Bend, the Dixie Highway was joined by the Michigan Road." When Indiana formed its state road system in 1919, the Main Market numbers became the first state road numbers, and this road was State Road 1 — the designation the corridor carried into the federal era. The SR 1 form matters to this section for one reason: it is the number the U.S. route replaced, on the same pavement, in 1926.
Sources: AARoads Wiki, U.S. Route 31 in Indiana (the March 7, 1917 signing and June 1, 1917 sign installation); Indiana Transportation History, "The First Five State Roads" (October 18, 2019), verbatim; the SR 933 route article (the SR 1 and SR 2 forms dating from the formation of the state road system).