HIGH-LEVEL CHANGES are afoot at American President Cos.

Matt Quartel, senior vice president of the company's Asian operations, has told the company he wants to retire at the end of 1992. The news has prompted a wave of speculation within and without of the company about who will take over the key position.According to tips from colleagues here at The Journal of Commerce, Michael Diaz has the inside track. He is the No. 2 man in APC's landside operation, APL Land Transport Services. But he's by no means a landlubber. Mr. Diaz's resume includes a stint as senior vice president of marketing and logistics for the ocean side of the company. He also served a tour as managing director of Taiwan.

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THE ASIA POST is a key slot for moving up within the company.

American President Lines, strictly a Pacific operator, needs a smooth- running Asian operation to survive. In addition, the intra-Asian business - moving goods from one Asian port to another - has become a booming area for the company.

In the meantime, no successor has been mentioned for Mr. Diaz in the land division. There is speculation that Timothy Rhein, head of the stack train operation, will assume Mr. Diaz's duties himself. That possibility is being taken two ways: Either Mr. Rhein wants to be more of a hands-on operator or the division is losing clout and no longer needs two top-notch individuals to run it. The landside operation was recently renamed, after all, to more closely reflect its association (some would say subservience) to the liner operation.

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LONGSHOREMEN AND SOUTHERN PACIFIC Transportation Co. are still sore about the situation surrounding the Los Angeles Intermodal Container Transfer Facility.

Some sources say the railroad is so mad it is considering taking the International Longshoremen's and Warehousemen's Union to court over damage allegedly done at the railyard before union members were ousted from their jobs. The union, which feverishly denies doing any mischief at the yard, said it had heard nothing about it from the railroad.

Despite grumbling about the performance of the facility's previous workers, the railroad has been cherry-picking the laid-off longshoremen. Railroad officials say they are interested in regaining some of the top workers in the previous work force. At the longshore protest rally in Los Angeles two weeks ago, union officials boasted that targeted members had rebuffed the SP overtures. Railroad officials say that since the rally they've seen several applications thrown over the transom.

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WHILE THE LONGSHORE BATTLE in Los Angeles continues, an older one in the north is ending - and the union is walking away.

When the USS-Posco Industries steel mill opened over four years ago, the longshore union complained bitterly about the use of steelworker instead of longshore labor to unload ships carrying raw steel to the facility. At one point, union members used private pleasure craft to block the Korean ships

from the river channel leading to the Pittsburgh-based mill.

After work-jurisdiction arguments were rejected by various courts, the longshore union raised environmental objections to the steel mill operation, claiming that the Korean ships violated local air pollution standards. The company, with a certain amount of spite, invested millions of dollars in anti- air pollution equipment for the ships.

Now, after more than two years of legal and bureaucratic wrangling, the union is dropping its lawsuit and agreeing not to make any claim on the steelworker dock jobs for at least 20 years, according to sources connected with the company. As part of the agreement worked out among the parties, USS- Posco is dropping its countersuit.