Deputy Superintendent Michael A. Cox of the Boston police reached an out-of-court settlement yesterday with one of the officers who allegedly beat him in 1995 after mistaking him for a suspect. Lawyers for Cox and the officer, David Williams, wouldn't disclose the terms of the settlement, citing a confidentiality agreement. A trial had been scheduled to begin Monday in US District Court in Boston to determine how much, if anything, Williams should be ordered to pay Cox. In January 1995, while working as a patrolman, Cox was dressed in plainclothes and was chasing a homicide suspect in Mattapan when he was brutally beaten by fellow officers, then left bleeding on the ground when they discovered he was a police officer. He suffered a concussion and kidney damage. In 1998, a federal jury found Williams and another officer, James Burgio, liable for using excessive force, but the case was closed after the city of Boston agreed to a settlement and eventually paid Cox $817,000. Cox's lawyers opted not to seek damages from Williams, Burgio, or a third officer, Ian Daley, who had been found liable for ignoring Cox's medical needs, because all three were fired from their jobs. But last year, US District Judge William G. Young granted a request by Roach to reopen the case and let a jury consider damages, after an arbitrator ruled in June that the department lacked ''just cause" to fire Williams and ordered him reinstated with back pay for the last six years. No criminal charges were ever brought against any of the officers suspected in the beating. [more]