Some insight into the correct exhaust shroud could come from the assembly manual. I do not have a '72 assembly manual, only '70 and '71. If the shroud is in the manual this means the part was installed at final assembly (Norwood). If not, then it was a pontiac engine plant installed item. 455HOs were not A or F model specific so when they left the engine line they were the same other than auto or manual trans version differences. If there was a running change from folded to rounded style I would expect that there would be a clean break as they ran out of the folded style. If the shroud is car-plant-installed the changeover date could vary by plant.
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