Others are more expert at this than I am, but as a general rule, any time you see Y with some numbers and letters after it, it's cheaper than full straight Y. For example, Y-26 used to be common on NW and something like YA03 means Y with three day advance purchase. There's also an "unpublished" government fare code that starts with Y and is treated as almost a full Y, with the elite upgrade happening overnight rather than at the time of booking and purchase.