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If you are talking about the indent in the sheet metal that sort of looks like a vertical vent slot I can only guess.
My guess is that the original rear end curve caused uneven detachment of air, resulting in drag from buffeting.
This small strip allows the air a clean sharp edge in which to detach cleanly.
A plan view of wind tunnel testing with a before and after would be able to confirm my best guess.
I do not think we will get this level of confirmation, however sometimes manufacturers provide little hints in videos posted on YouTube.
Do a search and see what you can dig up.
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