Former IBM marketing representative Edward Thomas Mann, the murder suspect in the IBM office building siege in Bethesda two weeks ago, unsuccessfully claimed in 1977 that the company denied him rapid promotions because he is black and gave preferential treatment and cash awards to a white woman doing the same work he was.

The D.C. Office of Human Rights investigated Mann's complaint, but former director James W. Baldwin dismissed it four months later, saying that "no probable cause has been found for crediting" it.

Mann's complaint, released yesterday by the agency after The Washington Post filed a Freedom of Information request, is the latest indication that the 38-year-old Mitchellville man became increasingly disenchanted with the giant computer firm before finally leaving IBM in December 1979.

Friends and acquaintances of Mann's said in interviews this week that he complained to them that he was not receiving the promotions he felt he deserved after working at IBM for more than 13 years. About 18 months before quitting he made a lateral job shift, switching from a technical job as an advisory systems engineer on a computer sales team to the marketing position he held when he left the firm.

When Mann left IBM, one friend said, "It was a mutual separation." Another source familiar with the case said IBM gave Mann $23,000 in severance pay.

IBM declined to answer questions about Mann's employment record and departure from the firm, except to say that "the determination of the D.C. discrimination complaint speaks for itself."

Mann allegedly drove his automobile through the glass doors of the IBM building near Montgomery Mall about 11 a.m. on May 28 and then sprayed about 150 gunshots throughout the three-story structure, killing three men and wounding six others. After more than 7 1/2 hours, he surrendered.

Court records show that five weapons were recovered from the office where Mann was hiding: an SM11-AL .380-caliber automatic machine gun, a 12-gauge Mossberg shotgun with a sawed-off barrel and stock, a Sterling .25-caliber automatic pistol, a Ruger .357-caliber revolver and a single-shot 12-gauge flaregun. In addition, police said they found more than 250 rounds of unused ammunition, a large portion of it on five bandoliers Mann allegedly brought into the building.

Mann said in his discrimination complaint he started at IBM in July 1966 as a systems engineer at an annual salary of $7,200 and was promoted to advisory systems engineer in April 1976 at a salary of $27,000.

He contended that IBM "has continually maintained a practice of disparate treatment towards black employes" and said that while it took him 10 years to win that promotion, it generally only took whites five or six years to advance.

Mann claimed that in 1976 he was assigned to "Demo 76," a marketing program in which two groups of workers performed similar tasks. He said a white woman was given a staff of "five experienced systems engineers", while he was give "only one inexperienced systems engineer."

In December 1976, Mann said in the complaint, he was given a lower performance rating than the woman and that as a result her group was given cash awards. "Since I performed essentially the same duties . . . with fewer people and received a lower appraisal, I am led to believe that race must have influenced my appraisal," Mann charged.

Archie Prioleau, a current IBM advisory marketing representative who describes himself as "a very close friend of Ed's," said Mann periodically expressed the feeling that IBM discriminated against blacks, but did so no more frequently than other blacks employed at the firm.

David H. Swann, a regional marketing administrator for IBM who occasionally had lunch with Mann while he was with the firm, said he never heard Mann voice discrimination complaints about the company. "I think basically he didn't feel he was getting promoted when others were . . . both blacks and whites," Swann said.

Mann is jailed on three murder charges and 11 charges of assault with intent to commit murder.