I have 11 LNAB unknown, 2 estimated births in the mid 1600's in England, 7 in the 1700's in England or Ireland, and 2 in the early 1800's in England.
They are all spouses of ancestors and so far I have not found marriage records that give the brides last name. Some baptism records give the mother's last name but none I have found for the nameless eleven.
I do check them regularly; it's quite surprising how often records are added whether on FS, FMP, FreeBMD or Ancestry. Sometimes additional information can be found on records for siblings.
It can be frustrating and takes time but occasionally I have found good sources by Google searching terms like Ellin wife of James married about 1650, Lancashire, England, or whatever the appropriate search terms are.