Weight codes used 1921-1971.
R Replacement (dates not certain; believed 1922-1959)
S pneumatic tire trailer (air-filled tire)
T solid tire trailer
Y Motorcycle dealer
Z Motorcycle; used 1921?-current
H confirmed 1921 to 1971
I confirmed 1921 to 1938
J confirmed 1930 to 1960
K confirmed 1924 to 1927
L confirmed 1934 to 1958
M confirmed 1931
N confirmed 1927 to 1956
O confirmed 1926
P unconfirmed before 1953
> P code from 1953 to 1965 used with variable weight, assumed only for Commercial
Q confirmed 1926
R single letter R confirmed as Replacement 1924 to 1926
single letter R assumed to be Replacement 1934 to 1936
used with weight/class code; confirmed 1936 to 1958
i.e., "RD" is replacement class D, passenger
"RX" is replacement Dealer.
S confirmed 1930 to 1935
T confirmed 1927
U&V Earlier years: Government-owned Truck or Trailer;
Later years: School Bus. School Bus believed to have started in 1948.
Research ongoing to determine which years each designation was used.
W unknown early years; possibly same as U & V
W Disabled (wheelchair) confirmed 1969 to 1977
X used 1921-current, except 1961 to 1965
Y confirmed 1964 to 1970
Z confirmed 1925-current; all-numeric exist in 1940's - assumed overflow after Z999
1937-1945 - all numeric plates assumed class C, presumably after C-99-999, went to 100-001
1964-1971 - 1 or 2 letters used, first letter denotes gross vehicle weight (GVW) of vehicle.
Below is a screenshot from "Reports and Resolutions of South Carolina to the General Assembly, Volume 2" from 1921 showing weight and class codes for license plates.
Passenger codes would remain in effect until 1971; Truck codes made changes over the years; Dealer and Motorcycle (X and Z) are still in use today.