SC 13 ALT: Appeared by 1940 as new primary routing running from SC 13 (now SC 124) west to SC 250 (now US 25) in Greenville. Shown but not labeled on official and county maps, but is explicitly mentioned in the 1946 SC Highway Department Annual Report. The road is also shown on officials as proposed to extend west to meet SC 13 just across a new Saluda River bridge into Pickens County.
It is unclear what happened to this route in 1947, as it was still only open to SC 250 when SC 13 was renumbered as US 123 in 1946. The proposed piece generally followed what became S-23-37 in 1946, and became US 123 about 1950 except for a small bit right off SC 250 which is S-23-1189 Old Easley Bridge Rd.

unlabeled SC 13 ALT west of Greenville 1940 Greenville County

SC 13 ALT route still there with US 123? 1948 Official