Postmaster General Preston R. Tisch announced yesterday the awarding of equipment testing and modification contracts to six firms, including a Texas company under investigation as part of a probe of corruption in the Postal Service.

Recognition Equipment Inc., of Irving, Tex., was among the recipients of contracts totaling $19.4 million, aimed at pushing forward the Postal Service's mail-sorting automation program.

REI is under investigation as part of a 14-month-old probe of corruption in the Postal Service. The company allegedly proposed a contract-sharing deal with a competitor, according to a report compiled by postal inspectors assisting in the investigation by the U.S. attorney's office in Washington.

Under the new contracts, REI, AEG, Bell and Howell, Toshiba and UNISYS will provide the Postal Service with new machines for testing next summer. A sixth firm, ELSAG, will offer a retrofit kit for equipment in use since 1982, the Postal Service said.

No one at REI has been charged in the federal investigation, but a criminal probe is being pursued, focusing on REI's campaign in 1985 and 1986 to win a $250 million mail-sorting contract, according to sources in and out of government. REI officials have said they were told last year by federal investigators that the company was not the target of the criminal investigation.