edoug,
You are correct-there were 3 non royal extinctions and Windsor.
Prior to that,the last non royal extinctions were Cleveland(1891) & Buckingham(1889).The royal ones were Connaught(1943) & Fife(1912) plus the deprivations of Cumberland and Albany in 1918.
As to the latter,s 2 of the Titles Deprivation Act 1917 reads:-
"It shall be lawful for the successor of any peer whose name has been so removed,to present a petition to His Majesty praying to have the peerage restored and his name placed on the Peerage Roll;and His Majesty may refer such petition to a committee of the Privy Council constituted as aforesaid(ie as set forth in s 1,which requires the presence of 2 judicial committee members);and should the committee be satisfied that such person has incurred no disability under this Act,and is well affected to His Majesty's person and Government,His Majesty may thereupon direct that the peerage be restored and the name of the petitioner be placed on the Peerage Roll;whereupon all rights and privileges of the holder of the peerage shall revive and be in force as if the name of the peer had never been removed from the Roll."
Now in the case of Albany,there is the argument that there is no successor,as none of the male heirs to the peerage complied with the Royal Marriages Act.However there is a logical inconsistency about this argument,given that adherence to a foreign power would bring the need for compliance with the Royal Marriages Act to an end,and the issue would only be whether the marriage was valid according to the domicile of the successor.