American Security Corp., holding company for Washington's second-largest commercial bank, currently is mailing to its stockholders an annual report for 1976 that represents a significant step forward in terms of more complete disclosure about its business.

For example, the report reveals that American Security Bank's trust and investment division unloaded major holdings of American Telephone & Telegraph Co., International Business Machines Corp. and three oil giants during 1976.

In the Washington region, American Security's trust division is one of the largest managers of money - with aggregate trust assets of $3 billion. About two-thirds of that amount is invested according to decisions of trust division officers and the balance is in custodian accounts without investment management responsibilities.

The decisions made by managers of such large blocs of investment funds are of obvious interest to other investors and to owners of American Security itself.

According to American Security chairman Carleton M. Stewart and president W. Jarvis Moody, their bank's expanded disclosure is based on the example of Bank of America, the nation's largest commercial bank. Stewart said he directed his associates to look over information Bank of America had decided on its own to provide for the first time, and to match the larger bank where possible.

"The inclusion of this additional data is a major step in the direction of fuller disclosure of corporate activities," the Amercian Security officers told stockholders.

In providing more details, American Security has joined a small but growing number of public corporations that have stepped beyond the letter of the law by disseminating to a broad public some information previously filed only with the Securities and Exchange Commission and some information not made public anywhere and not required.

Whether the new information is enough to meet demands of business critics who support broad disclosure of previously confidential data remains to be seen.

The trust and investment division figures provided by American Security, for instance, tell only part of the story. Take the decision to sell AT&T stock. According to the annual report, American Security sold the telephone company's stock in 1976 for $3,007,106. That's all the report says.

Looking into the range of AT&T trading last year on the New York Stock Exchange, a reader of the American Security report will discover that the telephone giant's stock was sold for as little as about $51 a share and as much as about $65. If one wants to make an assumption that $60 was close to an average for the year, that would indicate that American Security sold about 50,000 shares of AT&T in 1976. But that's just a guess.

Moreover, a one-year record doesn't indicate any longer-term trends. Perhaps American Security has been reducing its holdings of AT&T, on behalf of its trust clients, over a period of years. Readers of the annual report cannot get an answer from the data published but they will find out that whatever the trend, American Security continued to own a large bloc of AT&T at year's end despite sales that totaled $3 million.

A separate table shows that on Dec. 31 American Security owned 520,000 shares of AT&T out of 603 million telephone company shares outstanding.

All the reader of American Security's report can conclude, therefore, is that the bank's trust department sold a large number of AT&T shares in 1976 but that it retained an even larger bloc. In addition to AT&T, the report shows that American Security's other large stock sales in 1976 were:

Archer Daniel Midlands, $3,058,154; IBM, $2,546,142; International Minerals & Chemical, $2,338,210; Monsanto, $2,068,466; Philip Morris, $2,720,476; Phillips Petroleum, $2,520,429; Reliance Electric, $2,900,533; Standard Oil (Indiana), $3,057,762; and Standard Oil (Ohio), $3,577,681.

Similarly, the report shows the largest equity purchases last year as the following:

AMAX, $4,988,462; American Hospital Supply, $5,216,993; Becton Dickinson, $5,705,457; Burroughs, $3,589,132; Caterpillar Tractor, $4,090,007; Digital Equipment, $5,081,996; Heublein, $10,143,988; Union Camp, $4,877,374; U.S. Steel Corp., $4,857,940; and Whirlpool, $7,276,548.

American Security also lists the 25 largest equity holdings in discretionary accounts under its management, showing IBM, AT&T and Phillips Petroleum among the top 10, even though those three stocks were sold in some number last year. The only local company on the list is Southern Railway, of which American Security's trust department owns 288,000 shares (2.06 per cent of the rail firm's capitalization, the largest share in any of the 25 companies in which the bank has the biggest monetary holdings).

Other companies among the 10 largest holdings are General Electric, Exxon, Schlumberger, Burroughs, and Air Products and Chemicals.

American Security also lists the composition of managed assets by type of investment, showing common stock of $1 billion, or 51 per cent of the total; U.S. government bonds of $242 million (12.25 per cent); municipal bonds of $89 million (4.5 per cent); other bonds of $511 million (26 per cent); and the remainder in cash, time deposits, mortgages, real estate and preferred stocks.

In a separate section on the American Security "money center," which directs short-term investments for the bank itself, the annual report reveals that total interest and dividends earned on an investment portfolio last year was $16.4 million, 19 per cent of the bank's revenues from operations. None of the municipal securities was in default, and the largest holdings were bonds of California, Pennsylvania, Connecticut, Oregon, Maryland, Wisconsin, Ohio, Tennessee, Massachusetts and Illinois. More than 95 per cent of the securities of states or local governments were rated A or higher by Moody's Investors Service.

The Washington bank also reveals in its annual report that single-family residential mortgage loans approved last year were up 25 per cent over 1975. Of $197 million in residential loans on Dec. 31, 43 per cent was in Maryland. 41 per cent in the District and 16 per cent in Virginia. A separate table details all of the bank's loans by category and maturity.

As of Dec. 31, the report shows, loans totaling $8.7 million were past due on interest or principal payments, the report shows. Loans restructured to provide reduced and deferred interest and principal payments totaled $15.6 million. Total loans to real estate trusts were $15.8 million out of a total loan volume of $656 million.