Very little tangible information about MACV-SOG was in the public domain at the time of Apocalypse Now's release, and it would remain that way until the early 1990s. Its name and the fact that it was a covert action unit was broadly public knowledge, but little specific information was available; much like special operations forces today, much more was rumoured than actually known about SOG's operations. The unit's command histories — detailed accounts of operations and objectives for a given year — were eventually declassified in 1993, providing the first detailed insight into SOG's activities.

The Pentagon Papers — a colossal study of the Vietnam War commissioned by Secretary of Defense Robert McNamara in the late 1960s, and leaked to the New York Times in 1971 — contains extensive information about covert operations in Vietnam (for example, US incursions into Laos and Cambodia) in which MACV-SOG was certainly involved, but the unit's name doesn't actually appear even once in the 8,000-odd pages of the report.

Fragmentary information about SOG did appear in the press in the mid-1960s — often in connection with announcements of the deaths of servicemen assigned to the unit, or decorations awarded to them. The earliest reference is found in a dispatch from Vietnam published in the Philadelphia Inquirer and Washington Post in June 1965:

Headquarters buildings bristle with signs over doors that are about as pronounceable as the terms in a German engineering manual.

Some have an ersatz Scotch sound because they are prefixed with MAC for Military Assistance Command. Thus there is a MACSOG for Studies and Observation Group (South Vietnam must surely be the most overstudied and undercomprehended place on earth); a MACMAP for Military Assistance Program Directorate, etc.

That's a fine example of a deliberately innocuous cover name doing its job!

In 1969, amidst extensive coverage of the Green Beret Affair — the scandal surrounding the summary execution of Chu Van Thai Khac, a suspected double agent for North Vietnamese intelligence, by US Army Special Forces officers — some information about SOG's true nature began to appear in the American press. In an October 1969 report in the New York Times, reporter Joseph Treaster provided the most tangible insight:

In the sketchy reports on the recent Green Beret case, there have been repeated references to two secret intelligence-gathering units, Detachment B-57 and the Studies and Observation Group, or S.O.G.

[...]

Some dispatches from Saigon identified B-57 as a part of the Studies and Observation Group, an element of the American command. Subsequent investigation indicates that they are separate units. Both seek tactical intelligence, but S.O.G. also engages in sabotage and places emphasis on operations outside South Vietnam.

The Studies and Observation Group, which seems to have no less sensitive a mission than B-57, operates without cover from offices in central Saigon in the compound from which Gen. William C. Westmoreland once ran the war.

(Detachment B-57, otherwise known as Project GAMMA, was the unit implicated in the Green Beret Affair. Francis Ford Coppola would later confirm that Colonel Robert Rheault, the senior Special Forces officer at the centre of the scandal, was a partial inspiration for the character of Colonel Kurtz.)

In a June 1971 New York Times article reporting on the US' controversial cross-border incursions into Laos, we find likely the first accurate description of SOG's nature and the CIA's involvement:

American participation in the missions had come under a secret military unit known officially as the Studies and Observation Group. Established in 1964 as a joint venture of the Central Intelligence Agency and the Army, it has been involved not only in watching trails but also in attempts at rescuing prisoners and in other highly sensitive missions in Laos, Cambodia and North Vietnam.

In any case: regardless of how much was known about SOG specifically, there was plenty of reporting on US covert action in southeast Asia throughout the 1970s — thanks in no small part to several congressional investigations into the CIA and US military's activities in Laos and Cambodia, and the wider Church Committee investigation into the US intelligence community.