You would only have been able to SDC onto that flight if you had a ticket in Y or H class. The way SDC usually works for cheap fares is that somewhere within 24 hrs of the flight CO opens up all fare classes (even though pricing doesn't necessarily have fares that can sell them without advance purchase). If your fare class doesn't show up on Seatcounter (or whatever GDS tool you use) you won't be able to SDC onto that flight. the Y1 H1 probably meant that the flight was oversold, but at some point before departure seats for sale drop down to the capacity of the plane...they will no longer oversell. I think it's about 3 hours before, but don't remember.