SC 18 ALT #1: Appeared in Nov 1936 (SCDOT) as new BELT route running as a cutoff between SC 184 and SC 18 (now SC 28) in Antreville.
In 1938, SC 18 ALT was renumbered as SC 28 ALT. Today it is S-1-270.
SC 18 ALT #2: Appeared in 1938 as a renumbering of SC 111 ALT, running on Limestone St between US 29 and SC 18 Logan Av in Gaffney.
In 1940, SC 18 ALT became SC 18. Today it is the SC 18-150 CONN and a little SC 150.

SC 18 ALT #2
1938 Cherokee County

SC 18 ALT #3: Appeared in 1940 as a renumbering of SC 18 on Logan Av from Limestone southwest through downtown Gaffney to US 29.
In 1947, SC 18 ALT became the US 29-SC 18 CONN. North of SC 11 this is now part of SC 18 again.