US 21 ALT #1: Appeared by 1938 as a renumbering of mainline US 21 along Dogwood Ave through Ridgeway.
In 1947, US 21 ALT was downgraded to S-20-9 and SC 213 (now SC 34).
US 21 ALT #2: Appeared by 1938 likely as new BELT routing, running from US 21 west into central Rowesville, then north back out to US 21.
In 1947, US 21 ALT became SC 36 (now S-38-39) and S-38-21.

US 21 ALT Ridgeway Note 3 parallel roads shown as primary 1940 Fairfield County

US 21 ALT in Rowesville 1940 Orangeburg County

US 21 ALT #3: Appeared about 1939 as a renumbering of US 17-21 into Yemassee.
In 1947, US 21 ALT was downgraded to S-7-3 and S-25-1.

US 17-21 into Yemassee 1939 Official

US 21 ALT into Yemassee 1943 Beaufort County

US 21 ALT #4: Appeared about 1942 as new primary routing, running from US 21-301 (now SC 33) Orangeburg up Magnolia Ave to end at US 21/SC 2 (now S-38-94) in Stilson.
In 1948, US 21 ALT was renumbered as mainline US 21. Today this is now US 21 Business and US 601.
This US 21 ALT is explicitly mentioned in SCDOT annual reports as well as shown on a 1947 Orangeburg City map (shows two different US 21 routes).

US 21 ALT in Orangeburg - note there are two US 21s and two SC 210s marked, so one of each of those are ALT routes. 1947 City of Orangeburg

US 21 on Magnolia (note shield is left over from when it said 21 ALT) 1948 Orangeburg County

US 21 ALT #5: Appeared about 1948 as a renumbering of SC 554 around the east of Rock Hill.
In 1950, US 21 ALT south was truncated to the relocated US 21, leaving behind SC 72 ALT.
In 1953, US 21 ALT was renumbered as part of US 21.
US 21 ALT was explicitly shown on a 1948 City of Rock Hill map.

SC 554 around Rock Hill 1942 Official

US 21 ALT shown explicitly 1948 City of Rock Hill

US 21 ALT shortened from US 21 relocation to SC 5 1952 York County

US 21 replaces US 21 ALT 1953 York County