In the hushed atmosphere of a third-floor federal courtroom here, statisticians and business executives have, for weeks, been taking turns in the witness chair, reeling off sets of numbers, pointing to complex charts and discussing "IDPs," "AAs" and other corporate esoterica.

Behind the numbers and initials looms a massive legal contest between two giants--IBM and the federal government--over the thorny issue of racial discrimination.

In what some attorneys say could be a landmark case, the U.S. Equal Employment Opportunity Commission (EEOC) has sued IBM, contending it systematically discriminated against black professionals and managers in its nine data processing offices in Maryland from 1974 to 1981.

IBM, the computer colossus that ranks sixth on Fortune Magazine's list of the nation's 500 largest industrial companies, has denied the allegations, contending not only that it has not discriminated against blacks but that it is a leader in corporate affirmative action.

IBM says that between 1973 and 1981, for example, the number of black professionals and managers in the Maryland offices rose from 59 to 148, for an increase from 6.3 percent to 12.3 percent blacks in its professional work force.

In the shadow of the trial--and not part of the official record--is the burst of violence a year ago today when, according to police, Edward Thomas Mann, a black IBM employe who claimed the company made life miserable for minorities, opened fire with an automatic weapon in the lobby of IBM's Bethesda office, killing three persons and wounding seven.

A group of IBM employes called Blacks Against Racism issued a statement at the outset of the EEOC trial, saying they "have suffered in silence" over the years but some now are stepping forward, at the risk of their careers, to testify against the company.

"Unfortunately," the statement added, "one former IBM employe was so destroyed by his experiences that he destroyed the lives of others."

The trial here marks the first time the EEOC has taken on IBM in court.

IBM spokesman Daniel E. Udell says there have been 15 other (non-EEOC) discrimination complaints against IBM that went into court since 1972, and all were decided in IBM's favor.

Three other cases, he said, were settled out of court before trial. He would not say what the settlement terms were.

John Irick, chief EEOC attorney in this case, said in an interview that while IBM has a good equal opportunity program on paper, "it appears the managers and supervisors are not complying with it, especially in Maryland."

The nonjury trial is being heard by U.S. District Court Judge Norman P. Ramsey, who in the coming weeks will rule whether IBM has engaged in discrimination and should be barred from doing so in the future.

The action stems from a complaint originated in 1980 by former IBM employe George W. Hunter Jr., who contends his initially successful career was blunted when he was denied a promotion to a branch manager's position in late 1974.

At the time he was an administrative assistant to an IBM division vice president, a position from which employes traditionally are boosted to branch manager, Hunter maintains.

Instead, Hunter said he was offered a "location" manager's spot in Providence, R.I., a position subordinate to the branch office in Boston with fewer employes and responsibilities.

He refused that offer, failed to get a branch manager position thereafter and eventually felt compelled to resign from IBM in 1977--all because of his race, he said.

Hunter said he was the only black administrative assistant (AA) at the time and the only AA who did not go next to a branch manager position or its equivalent.

EEOC took his case, broadening it into a class action.

The basic allegation is that the salaries and promotions of blacks lagged behind those of comparably qualified whites because of racially prejudiced "performance appraisals" by predominantly white supervisors.

About a dozen black employes have testified, claiming they were denied various promotions and salary increases.

William E. Sedlacek, a statistician testifying for the EEOC, concluded there is significant evidence that blacks are rated consistently lower than whites on their appraisal ratings--the basic document that determines promotions and salary increases--and even those blacks with seniority and ratings comparable to whites are paid less.

IBM statistician James Medoff countered that there were no gross disparities in promotions and salary advancements.

Other witnesses testified that because the number of blacks hired at entry level increased steeply with affirmative action programs in the last 15 to 20 years, blacks at any given time are younger and have less seniority on the average than whites, thus giving a superficial appearance of racial lag.

Hunter, who worked at IBM from 1966 to 1977, received 14 pay increases in that period and was receiving an annual salary of $39,600 when he left, said IBM, putting him in the top 3 percent of persons in comparable positions.

Charles E. McKittrick Jr., the division vice president for whom Hunter was an AA, testified he eagerly sought Hunter as his assistant in early 1974 because he "looked good in comparison to other candidates . I also was--all things being equal--quite anxious to have a black assistant."

He said Hunter's "IDP"--individual development plan--showed him to be promotable to branch manager, and in late 1974, McKittrick said he put Hunter on the promotion list, deleting a white candidate he deemed less qualified.

At the time, however, IBM was cutting back the number of its branch offices, McKittrick said, and he was able to offer Hunter only the Providence "location" office.

EEOC attorneys countered that even with the alleged shrinkage in operations, numerous branch office appointments were made among Hunter's white co-workers from 1975 until his resignation in April 1977.