P-51 Exhaust stacks

I've noticed that Allison-powered Mustangs generally have unshrouded exhaus t stacks, while Merlin-powered Mustangs can be seen with both shrouded and
unshrouded exhaust stacks. Was there an operational reason for this differ ence or was this a matter of different versions of the Packard Merlin or di fferent Mustang production blocks?
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