43X168xxx
4 = Buick
3 = 1973 model year
X = Fairfax, KS
168xxx = sequential number, range for Fairfax was 100001 to 200982 for 1973 (this would match the last 6 digits of the VIN of the donor car, factory replacement engine usually start with RE, so this appears to be out of another car)
F 442 = engine production code (there is probably an X in front of it making it XF 442)
X = 1973
F = 455 w/ 4V carb
442 = date code
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