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I would think it is almost certainly for noise reduction, the OEMs don't increase costs with things like that unless they are forced to do so by regulations!
Although I wonder if it may remove the boundary layer thus changing the main gas flow closer across the other pipe for better venturi effect?
I'm with you on the expansion slot, you sometimes get the same thing on the manifold flange between the primaries if the head and manifold have different expansion properties - it stops everything disintegrating when it gets really hot.
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