I see now.. I was adding the serial numbers but SDK wouldn't see them until I updated the draft
oDraft.Lines.SerialNumbers.InternalSerialNumber = "123456";
oDraft.Lines.SerialNumbers.ManufacturerSerialNumber = "Ga-bage";
oDraft.Lines.SerialNumbers.Add();
oDraft.Lines.Add();
oDraft.Update();
retval = oDraft.SaveDraftToDocument();
I've yet to play with oDraft.Lines.SerialNumbers.Add();,oDraft.Lines.Add(); to see what effect their omission will have..
Thanks again Edy!