Of all my 6-series machines (8 of them), this is the only one with a date code sticker:

Smith-Corona Classic 12 (1975)
S/N 6LTV 452851H
Date Code HT31327

This next one is not strictly a 6-series machine, but may help in figuring out the SC date code:

Smith-Corona Sterling Automatic 12 (1976)
S/N 3LRC 374127
Date Code SF22730