Ducommun Inc., Los Angeles, reported that it had signed a letter of intent to acquire Torrance-based Theim Industries for about $21 million . . . Cleveland-based TRW said it expects its first-quarter earnings to be lower than the $1.57 per share it earned in the first quarter of 1984 . . . Atlanta-based Delta Air Lines said it flew 2.95 billion revenue passenger miles last month, up 21% from March, 1984 . . . Trans World Airlines, New York, reported that it flew 2.76 billion revenue passenger miles in March, up 50.6% from a year earlier . . . A federal judge in New York granted an extra 30 days for the securities and life insurance industries to negotiate a financial aid plan for investors who bought Baldwin-United annuities.