Every now and again, particularly in the PDF files covering
equipments, you will see a scanned page bearing a Security Caveat.
None is higher than RESTRICTED so there is nothing shocking in terms of
security lapses. Moreover, with only a very few exceptions,
documents so marked were given that security marking in the 1950's.
Thus, it is reasonable to believe that some fifty years on, the
lowliest of the classifications, viz, Restricted, is now de-classified
to the level of "public domain" awareness.