Robert A. Moon, a career postal employee who in 1963 won a 20-year fight for what was to become the ZIP code, died on Tuesday at a hospital in Leesburg, FL, 34748. He is the undisputed father of the first three digits in the code.

Mr. Moon was 83 and lived in a nursing home in Leesburg.

The Zoning Improvement Plan, as ZIP was more prosaically known, represented an irreversible step in the seeming ascendancy of numbers over letters, arriving about the same time the telephone company was switching to seven-digit numbers from letter exchanges like BUtterfield 8. Newspaper editorials warned of Big Brother and ''numerical neurosis.''

But ZIP codes made the mail zip at a time when old delivery systems were breaking down. Planes carried far more mail than trains, on which leisurely sorting on mail cars took place, and the volume of business mail was ballooning. Also, the mail was going through increasingly mechanized regional centers, rather than city to city, ruling out old-fashioned hand sorting by city and state and neighborhood. The Postal Service, then the cabinet-level Post Office Department, needed a national code to make sorting as fast as possible.

Of perhaps even greater long-term importance was the ease -- initially unforeseen -- with which mass marketers would use ZIP codes to pinpoint demographic groups, like areas with high incomes or many people who apply for credit cards.