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Those are series numbers.
A 442 could come in a convertible 4467 or hardtop 4487; both are 4400 (think of it as 44XX) series.
5400 is a Delta 88, which could come in hardtop sedan 5439, hardtop coupe 5457, or town sedan 5469; again all 5400.
The model series numbers varied by model year - for example for 1968-71 the 4400 series was the 442. Note that it doesn't exist for the 1972 model year since the 442 was demoted to an option package, so the 4400 cars don't show up on this VIN chart for 1972. For model years 1965-71, the model series are the second and third characters of the VIN. Starting with the 1972 model year the VIN format was changed to replace these two digits with a single letter to make room for the engine code in the fifth position. The Fisher Body Style Number found on the cowl tag will start with these two digits also.