Historical version 2 of Earl's Court But Barons Court (view current version)
At least according to tube stop names, Earl's Court has an apostrophe, but [Barons Court]? doesn't. Apparently, the latter probably had an apostrophe at one point, but it got lost:
http://www.linguistlist.org/~ask-ling/archive-most-recent/msg04651.html
And let's not even think about St James's Park ...
See also: Easily Made Mistakes.