The controversial note allegedly came from the "death cell" of the Kei road police station and it was addressed to South African security policemen, Capt. Petrus Schoeman.

"This is just to say goodbye to you. You can carry on interrogating my dead body. Perhaps you will get what you want from it. Your friend, Mapetla."

This epistle, written on a piece of toilet paper, is at the center of an unusual court case in this staid university town that may allow black activists here to strike back at police brutality and also challenge the government's widespread practice of detention without trial.

Police are claiming that the note is proof that Mapetla Mohapi, the first prominent activist in the black consciousness movement to die in police custody, committed suicide by hanging himself from the window bars of his cell with a pair of blue jeans in August 1976.

Mohapi's wife, Nohle, says the note is a fake and is part of a police coverup of the fact that they killed her husband. Clarence E. Bohn, associate professor of forensic science and document examination at George Washington University in Washington who has 35 years of FBI experience, backed up Nohle Mohapi's charge by testifying recently in a superior court here that the "suicide" note is a forgery.

Although the case had not attracted the international publicity that surrounded the inquest into the death of Mohapi's fellow-activist and black consciousness leader, Steve Biko, who died 13 months after his good friend, the proceedings are significant for several reasons.

It is the first case in which the police have produced a note to explain or prove one of the numerous "suicides" among the 44 people who have died in police custody since 1963 when the first detention-without-trial law was enacted.

It is also the first time the police's version of how a detainee died is being scrutinized in civil proceedings initiated by the family. In four previous cases, family challenges of the police's explanation have been settled with relatives out of court. Two cases occurred in 1969 and two this year when the government gave $18,000 to the widow of Joseph Mdluli and a record $78,000 to the Biko family. Nohle Mohapi is claiming $42,000 in compensation.

Finally, if the family successfully convinces Supreme Court Judge Johan Smalberger that the note is a forgery and that the police story lacks veracity, it will open the way for criminal action against the policemen involved, according to Mohapi's lawyer, Griffiths Mxenge. Only once have security policemen been criminally prosecuted for the death of a prisoner, but on that occasion the four officers accused of Mdluli's death in March 1976 were acquitted.

Government sensitivity about the South African reputation for brutality produced by frequent deaths of people in police custody has resulted in a drastic reduction of such occurrences since Biko's inquest. Between Mohapi's death in 1976 and Biko's more than a year later, police were reporting an average of one death a month. But in the last two years only two men have died in security police custody. In addition, the government has announced a review of its security legislation that includes the Terrorism Act's notorous "Section Six," which allows indefinite detention in isolation purely on suspicion of crimes.

Government critics are doubtful that the review will produce a return to a habeus corpus rule at a time when the authorities are more concerned than ever about the threat of subversives. These critics also note that with this machinery intact, deaths in detention might resume if black dissident political activity again reaches the level of 1977.

Mohapi, who was 28 when he died, was a graduate in social work from the all-black Turfloop University. He was an official of the black consciousness-oriented South Africa Student Organization (SASO) and the Black People's Convention (BPC). After five months of detention without trial in 1975 he worked for the Zimele Trust Fund, an organization he and Biko founded to help former political prisoners and their families.

In 1975 Mohapi went to Durban to work as an organizer for the black consciousness movement but was banned by police from political activity and sent to Kingwilliamstown where Biko also lived. He was detained again by the security police on July 16, 1976. At 9 p.m. on Aug. 5 police arrived at his home to inform Nohle he had committed sucide.

When asked if he had left any note to explain his action, police said no. But five months later at the official inquest into his death they produced the now-disputed note. Mohapi's wife, who was detained without trial for a year after his death, was more than a little suspicious of the police's suicide note.

"I do not for one moment believe that Mapetla would have left a note for the police and not for me," she told a local paper in 1977. "I am the only person he would have thought of first."

In fact, what the police at that time did not know, was that Mohapi had smuggled three notes out of prison to his wife and mother-in-law before he died. Rather than a depressed state of mind, they reveal that he was optimistic about his future.

"My dear wife," he wrote in one, "I have confidence in you that you will look well after my kids until I am back, although I do not know when." Mohapi has two daughters, Motheba, 5 and Konehali, 3.

Other evidence at the inquest threw more suspicion on the police account. Black journalist Thenjiwe Mtintso, a colleague of Mohapi, testified that while she was detained by the same security police who had taken Mohapi, they demonstrated to her how they killed him.

She said that Maj. Richard Hansen took her to the Kei road police station and he "had a big wet towel with him. . . . He told me to sit on the floor . . . He put the towel over my head. Then he pulled the ends over my neck. It made me feel I could not breathe." t

When he took the towel away, Mtinsto testified, Hansen said, "Now you see how Mapetla died." On another occasion Mtintso said Capt. Schoeman told her she would go the same way as Mapetla.

In addition, the constable who found Mohapi's body testified suicide was inconsistent with the Mohapi he knew and that the prisoner had not appeared depressed. Two family doctors, who attended the post-mortem with a government doctor, were detained by police a few days afterwards, the inquest was told.

Nevertheless, the conclusion of the inquest was that Mohapi died from suffocation caused by hanging and that no one was responsible for his death. The magistrate, however, did not deliver a formal verdict of suicide nor make a finding on the authenticity of the suicide note.

The family's civil claim for damages from the state has reopened these questions. And because the superior court proceedings offer more legal latitude than the lower court death inquest, the family hopes that different conclusions will be reached.

The police are expected to produce their own handwriting analysts to say the note is genuine. The case has been adjourned until Feb. 4.

Many blacks have hopes that the case will help a settle a dispute over detention without trial. "We have been saying all along that people detained under "Section Six" are brutalized by the police -- who deny it," said lawyer Mxenge.