Christmas Island has been an Australian territory since 1958 and remained postally and philatelically independent until 1993 when Australia Post took over postal and philatelic services for the island. Since then all Christmas Island stamps are valid in Australia and vice versa. Fore more information on the postal history of Christmas Island see this wikipedia article.
SG : Stanley Gibbons Catalogue Number, please note for some items such as gutter pairs a suffix is added for ease of reference but these items are not included in the catalogue.
MUH : Mint Unhinged or Mint Full Gum is used to describe stamps with full original gum as issued by the post office.
FDC : First Day Cover