differentiating Buick, Olds or Pontiac ( BOP ) is easy-peasy.
http://www.teambuick.com/reference/ident_eng_visual.php
4 5 H 420018
4 = Buick
5 = 1975
H = Flint, MI production plant
420018 = sequential production number, matches the last 6 digits of the VIN from the car it came from. not very useful if you don't already know the car.
Buick only made 2 v8s in 1975, the sbb 350 ( 6 bolts in valve covers, VERY different than sbc 350 ) and bbb 455 ( 5 bolt valve covers ).
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