SC 19 Trenton to Whitmire 1926 Official

SC 121 Trenton to Whitmire 1964 Official

SC 72 Whitmire to Rock Hill 1950 Official SC 72-121 Whitmire to Rock Hill 1964 Official

Adjustments: In 1982, SC 121 was removed from Reynolds Rd and abandoned routing east of the railroad north of Trenton.
Improvements: Paved since creation
Upon inception, SC 121 was 4-laned: North Augusta to S-2-33; Whitmire to US 176 WB; the brief former US 21 Bus multiplex south of Rock Hill
The Chester Bypass is shown as 4-lane in 1977.
SC 121 was 4-laned from I-20 north to about S-19-429 in 1978.
The Rock Hill Bypass above SC 5 was 4-laned in 1989 or 1990.
In 2010 or 2011, SC 121 was multilaned from SC 395 to US 76.
About 2014, SC 121 was multilaned from SC 5 southwest to SC 72-901.
Comment: SC 121 was created as a 3-state route. Was this meant to be US 121 at some point? Too late, now, with the designation of US 121 in VA-WV. While South Carolina could probably use a US Route that runs along SC 121, Georgia and Florida's parts are already surrounded by US Routes. Perhaps an extension of US 319 would make sense here.
The SC 72-121 multiplex is easily the longest current multiplex in the state, though the now defunct US 17-701 route was a little longer, and SC 2's multiplexes up to 1948 were quite lengthy as well.

SC 121 Aiken to Columbia via Wagener 1926 Official

SC 215 replaces SC 121 1930 Official

SC 121 #2: Appeared in 1929 as new primary routing, running from US 21 Branchville northeast through Bowman to end at SC 31 (now US 176) Providence.
In July 1937 (SCDOT), SC 121 was extended east as new primary routing to end at US 15.
In 1940, SC 121 was extended with US 21, then west with US 78 into Bamberg County, then replaced SC 65 ALT to SC 65 (now SC 61), then continued as new primary routing to end at SC 362. Also, SC 121 was extended east as new primary routing to SC 6 Vance.

SC 121 Branchville to SC 31 1930 Official

SC 121 extended to US 15 1937 Official

SC 121 expanded to run SC 362 to Vance 1940 Official

In 1947, SC 121 west was truncated to US 21 Branchville, leaving behind US 21, US 78, S-5-10 (now S-5-18), and S-5-18. SC 121 east was truncated back to US 15, leaving behind S-38-40.
In 1951, SC 121 east was re-extended to SC 6 Vance over S-38-40.
In 1964, SC 121 was renumbered as SC 210.
SC 121 lasted long enough to have an interchange with I-26.

SC 121 truncated to run Branchville to US 15 1947 Official

SC 121 extended back to Vance 1951 Official

SC 210 replaces SC 121 1964 Official