What's an "outrigger" for parking aircraft on a carrier?
You can see it a bit better in this picture: 



(Source)


  In order to save deck space for operations, the smaller escort
  carriers like HMS Khedive used outriggers. The tail wheel is first
  affixed and locked to the rigger trolley and then the aircraft is
  pushed outwards and the wheels heavily chocked fore and aft of the
  wheels. Here one of Khedive’s Corsairs demonstrates the concept
  perfectly. Note that the Corsair has Type C roundels, indicating it is
  in the European theatre of operations.


They were effectively a simple way to save deck space with out enlarging the deck. There is another decent pic here and some spitfires on outriggers here

Based on the info here they can be found as far back as the late thirties


  It represents an F4B-4 positioned on an experimental outrigger aboard
  the USS Saratoga during the late 1930s. The Saratoga and several other
  aircraft carriers were selected to try out different types of deck
  outriggers for the Navy in the interwar era.