It was super-sensitive, hush-hush stuff: armed guards, double-locked doors, drawn shades, electronic bug detectors, computers with scramblers and disconnected telephones.

Sounds like someone's war room, but it was about corn and soybeans, lemons and pears, hops, filberts and other delectables from America's farms.

This cloak-and-dagger exercise began around 3:30 one morning last week when Wil Walther and Gerry Clampet, unprepossessing Agriculture Department bureaucrats, met and headed directly with an armed guard to a double-locked vault in the basement.

As the guard watched, Walther and Clampet opened the safe, took out a stack of envelopes and rode an elevator to the fifth floor. Once there, they went through a double security door and down a hall to a room where the envelopes were opened and their contents reviewed.

The envelopes were from state offices of the USDA's Statistical Research Service (SRS). They contained crop production estimates, gathered during August by SRS technicians, that would be combined into the department's big September report on the state of U.S. agriculture.

The SRS regularly produces reports, using many of these high-security steps, on everything from cherries and citrus to hogs, turkeys and mushrooms. But this month and last there has been heightened interest in the SRS Crop Reporting Board's findings because of a drought that has devastated much of the country's grain production.

In the volatile markets accompanying the drought, fortunes can be made by traders, grain companies, speculators and farmers who play their cards right. In this atmosphere, information is power. And power could come from knowing how the USDA will assess production nationally.

So in the interest of fair play, to assure that no one has advance notice of the crop data, the USDA goes to extreme lengths to guard its information: secrecy oaths for SRS employes, armed guards, double-locked doors and sequestered printers.

The board's activities go on in a fifth-floor corner of the Agriculture Department's South Building here that is known around the department as the "lockup." Once an employe or visitor is cleared for access to the lockup, departure is barred until that day's crop report is released to the public--3 p.m. last Monday, in the case of this month's report, after commodities markets were closed.

SRS employes joke a bit about the tight security, but they don't take it lightly. When Secretary John R. Block made his first visit to the lockup, he was refused entry because he had lost his official pass. He finally got in when he showed a White House pass that confirmed his identity.

A soft-drink truck driver wasn't quite so lucky recently. He got into the lockup to service his pop machines, but he was required to spend the day inside the secure area while his truck remained double-parked on 14th Street.

"We exist for the benefit of the people who are doing the farming out there," said SRS Administrator William E. Kibler. "We must be objective and accurate . . . . We have to maintain the confidentiality of reports from farmers . . . . There must be no leaking of data."

From time to time, there have been allegations of breaches of the USDA security system, but the only one confirmed occurred in 1905. That involved an employe who sent cotton information to an accomplice outside by raising and lowering a window shade. The signals were misread; money was lost, and one man ratted on the other.

That blatant but maladroit stunner left an indelible mark on the people who put together the crop statistics. Security is tight and getting tighter. In 1965, for example, the USDA got the National Security Agency to devise a code for transmitting crop data by Telex and telephone. More recently, a computer and word processors have been secured and made tap-resistant.

The SRS has established a system that quickly digests the data sent in from the field, produces the crop estimates for the entire country and prints it out, ready for release to the public--all in less than half a day. Over the years, these estimates have been remarkably accurate.

On days when reports are released, SRS officials take turns sitting as members of committees that make forecasts for individual crops. As chairman of the board, Walther sits on each of the committees and settles any major differences. As each panel reviews state numbers and comes up with its forecast, it is added to the draft report and readied for the printers.

Then, before the 3 p.m. release time, Block comes into the lockup and signs the report. If he doesn't make it on time, Kibler and Walther release the report anyway.

Maybe the best and the zaniest comes last. Just before 3 p.m., Walther and Clampet put copies of the printed report, face down, in telephone booths that line two walls of a small room outside the lockup. When the tamper-resistant wall clock (it's encased in a tough plastic box) says 3:00, Walther says "Go!"

Wire-service and commodity-news reporters poised behind a white starting line woven into the carpet rush to the phone booths, flip open the reports and breathlessly call their home offices with the board's findings. "Bulletin" and "urgent" messages clatter from news tickers around the country. Finally the world knows.