>Executive Directive 51 looks like it greatly expands on Ex. Ord. 11490
Executive Orders must be printed in the federal register. The following do not have to be made public or even admitted to exist.
National Security Council memos were called National Security Action Memorandums under Kennedy, Bush changed the name again to : National Security Directives.
NSD's are powerful and dangerous tools.
William Cooper - Behold a Pale Horse , chapter five ; Balance of Power