The Federal Communications Commission yesterday said it would open its files to American Telephone & Telegraph Co., the Bell operating companies and MCI Communications Corp. to settle allegations that FCC decisions in the 1970s were tainted.

AT&T and the Bell operating companies have alleged that decisions concerning long-distance competition made in the 1970s by the FCC may have been unduly influenced by MCI Communications Corp. and that ex parte contacts between FCC members and MCI may have taken place prior to decisions benefiting MCI, the FCC said. AT&T is concerned that the actions may have increased its liability in antitrust suits brought by MCI charging that AT&T had illegally stifled its first long-distance competitor.

FCC sources said AT&T has brought to light billing records indicating a meeting between MCI lawyers and FCC officials; personal notes made by Ken Cox, general counsel to MCI, about the meetings, and a draft of a 1974 FCC document ordering AT&T to provide crucial interconnections to MCI that AT&T says was found in the files of William McGowan, chairman of MCI.

"We can't fathom why something that was . . . available years ago has suddenly become an issue," said MCI spokesman Gary Tobin. AT&T declined comment.

In a separate action, the FCC said it would not appeal a decision last month by the U.S. Court of Appeals for the District of Columbia that voided a long-standing FCC rule that cable television systems must carry every local station broadcast in their area. Agency "must-carry" rules will be voided Sept. 3, the FCC said.

As a result of the court decision, the FCC will support changing copyright laws as they apply to cable companies and television broadcasters "because they have gone hand-in-hand with must-carry rules," said FCC general counsel Jack Smith.

Currently, cable operators have the right to carry any TV signal without permission, in effect allowing for retransmission of copyrighted material. Cable operators pay a nominal royalty of 3.75 percent of revenue into a copyright pool distributed to broadcasters and program producers for long-distance signals only.