A SEVERELY mentally retarded patient with no personal history and known only as No 530 in Castle Peak Hospital over the past decade died after a mysterious incident last year, an inquest heard yesterday. The man, aged between 40 and 60, had a mental age of less than three. He had been found wandering in Tai Po on December 3, 1982, unable to give his name or particulars. To the mystery of 530's life was added that of his death, as the fellow patient who might have testified about how he received his fatal injury was unfit to give evidence because of severe mental retardation. The Coroner's Court heard that 530 was found lying unconscious on the floor by a nurse, Kwong Chi-kit, 32, at 8 pm on June 2 last year. Mr Kwong, who was taking evening meals to patients, said he saw the man had an injury to the back of his head and he immediately tried to resuscitate him. Soon afterwards, 530 was transferred to Tuen Mun Hospital where he remained in a coma for 21/2 months, finally dying on August 28. Mr Kwong testified that he had heard from other patients that the deceased had been pushed to the ground that day by Leung Ping-tim, who had a mental age of 31/2 and was unfit to testify. The nurse's superior, Kenneth Tang Chi-keung, 37, said Mr Leung had admitted pushing 530. He said 530 was quarrelsome and had frequent altercations with other patients. In his summing-up, Coroner Warner Banks said evidence regarding the incident on June 2 was only hearsay which could not satisfy the higher standard of proof for unlawful killing. A three-member jury returned a verdict of accidental death.