The Special Operations Executive. WWII clandestine operations agency absorbed into SIS towards the end of the war. Security-edited records from the period before absorption have been released by SIS to the UK National Archive.
Found on https://www.sis.gov.uk/glossary.html
Soe noun [ Scot. sae , say , saye ; confer Icelandic sār a large cask, Swedish s... a tub.] A large wooden vessel for holding water; a cowl. [ Obsolete or Prov. Eng.] Dr. H. More. Found on http://www.encyclo.co.uk/webster/S/137