From Lee Russell- Tour of Duty military advisor 1987-1989:

This is just a mistake of the show's original writers. During the Vietnam War the Army made little use of the rank of corporal. It was generally reserved for "busted" sergeants. In 1967, in the pay grade of E-4, the Army had 220,000 specialist 4s, but only 2,000 corporals. In any case, it would take about a year for a good soldier to reach E-4, and Percell has not been in the Army this long.

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