On March 25th, 1948 the Army General Staff, namely the Chief, Intelligence Group, Colonel Riley Ennis authored this directive, on behalf of the Director Intelligence. It was a request for reliably reported sightings of Flying Discs. The reliable reports of these discs were to now fall under the heading of Unconventional Aircraft.
Though today, we could find items such as parachutes, hang-gliders and large kites as possibly being described as Unconventional Aircraft, on March 25, 1948, Colonel Ennis made his wishes clear, but stressed experienced personnel as witnesses being the vital or crucial issue.
Control No. A-1917
Bob Note, As per line number five:
If everything had been balloons and misidentifications there wouldn't be a directive in the first place.
452.1 seems to be one of the main filing system designs which has been used by the General Staff, for National Security Sightings of Unconventional Aircraft. I have seen 452.2 and 333.1 also connected to this subject.
This Interesting Curiosity, downloaded from the CIA on-line FOIA Labrary, appears to be from Deputy Director Wright's Jourals and intinerary. It will remain in the "open" basket until a copy can be obtained which has not been so redacted.