SC 19 ALT #1: Appeared in 1939 as new primary routing, running from US 25 northeast across SC 19 to Trenton, then back over to SC 19.
In 1947, SC 19 ALT south was truncated to SC 19, leaving behind S-20-29.
It appears that in 1955 or 1956, SC 19 ALT was moved from Watson/Church Sts to use Wise St all the way through Trenton. Wise St through Trenton may have been a separate SC 19 ALT prior to 1947. Wise St became S-20-149 and Church St became S-20-19. This is shown with original concrete on 2012 GMSV.
Between 1965-67, SC 19 ALT was renumbered as SC 19-121 CONN. The 1964 Edgefield County map does show 19 ALT still throguh Trenton when SC 121 replaced SC 19 north of Trenton.

SC 19 ALT appears 1940 Edgefield County

SC 19 ALT truncated to SC 19 southeast of Trenton 1951 Edgefield County

SC 19 ALT moved to Wise all the way 1956 Edgefield County

SC 19 ALT becomes SC 19-121 CONN 1967 Edgefield County