The SEF is a 15'6" w/b *, closed cab, belpaire box, narrow tank, early Collett (1924) bunker, Churchward smokebox, can be built with or without rear outside boxes.
Essentially:
http://www.gwr.org.uk/notes/4c-517L.gif
A good 517 thread:
http://www.rmweb.co.uk/community/index.php?/topic/92797-george-armstrongs-masterpiece/
* edit: not completely sure of this yet, the kit might cater for the 15' w/b.