March explained

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The three numbers defining March models were simple to understand to anyone who knew the formulae of the time. The first two numbers, 71, 72 etc, were the model number and indicated the season they were to be raced, 1971, 1972 etc. The third number or letter showed the formula for which the car was designed, 1 - formula 1, 2 - Formula 2, 3- Formula 3. The letter B, referring to Formula B, or Formula Atlantic. Sometimes a extra letter was added; this indicated the engine option; M - for BMW engine, S - for sports car.
Examples:
March 711 was the first Formula One car built for the1971 season.
March 712 a Formula 2 car for the 1971 season.
March 742 a Formula Two car built for the 1974 season.
March 74B a Formula Atlantic (B) car built for 1974 season.
March 752 a Formula 2 Car built for the 1975 season.
March 753 a Formula Three car built for the 1975 season.
March 75B a Formula Atlantic (B) car built for the 1975 season.
March 75S a 2-liter open sports car built for the 1975 season.
March 782 a Formula 2 car built for the 1978 season.
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