The (single) Merlin 1D vacuum engine's exhaust plume has a symmetric shape, comprising of six distinct "inner" plumes. Why does it have this shape?

Is it due to the shape/configuration of fuel injectors, or does the turbopump exhaust shape the plume somehow? Or something else entirely?

As seen during the recent SAOCOM 1a launch:

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    $\begingroup$ I believe it’s due to the configuration of the pintle injectors, but it’s hard to be sure since the schematics aren’t public $\endgroup$ – Jack Oct 9 '18 at 9:21
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    $\begingroup$ Turns out that Scott Manley has a video on the SAOCOM launch and he thinks this is due to the slit-type pintle injectors producing uneven pressure areas. Also, from his video it seems the "inner plumes" aren't actually symmetric. $\endgroup$ – czert Oct 9 '18 at 10:18

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