The "L" prefix began in 1968, and continued up to about 1974 when it was discontinued. Values in the SCSW4 are Exc= 550, VG=475, Good=425.
Values of this model have not gone up much, I paid 450 for one in Exc condition back in 2007, and that's just about what it is still worth today. Mine s/n L13xx, was shipped June 1972.