US: Director General of the Security Service PS letter to No.10 ("Forged Recording of a Telephone Conversation Between the Prime Minister and President Reagan") ["there is no information to indicate that any subversive group or individual in this country was involved in making this tape"] [declassified 2014]

"PO Box 500" refers to the wartime address of the Security Service, colloquially known as MI5, Sometimes in government it is referred to as Box 500. The letter reference F/195/DG/PS suggests the letter was written by the Director-General's Private Secretary. The full file from which this item is taken can be seen here.