Yes: I believe you are correct.
I haven't (yet) actually assembled and successfully tested such a cabling setup
(I'm still building it in my mind's eye
I'm going on my knowledge of RAID controllers and cabling, in general.
The SFF-8087 connector is wider with 2 channels on one side,
and 2 channels on the other side of the contact plane.
The SFF-8643 connector split that contact plane down the middle,
and stacked the two "halves" to create a denser connector
(meaning, a narrower housing). This is the cable end
that mates with the corresponding ports on the model 3840A RAID controller.
At the other end, the SFF-8639 got a new name -- U.2 --
which is easier to identify because it mates with a more elaborate connector
on 2.5" NVMe SSDs
There are plenty of photos on the Internet of this elaborate connector.
(I believe "U.2" was chosen as the companion of "M.2" SSDs,
i.e. much easier to remember.)
Also, U.2 connectors typically come with a DC power input connector,
e.g. either SATA-style or Molex-style power input.
When I don't have the parts in-hand,
I like to search the Internet for photos of each e.g.:
Google SFF-8087 cable photo
Google SFF-8643 cable photo
Google SFF-8639 cable photo
Highpoint's specs list the SFF-8643 connector here:
http://highpoint-tech.com/PDF/RR3800/RocketRAID_3840A_PR_16_08_09.pdf
see "Four SFF-8643" (on the RocketRAID 3840A NVMe RAID Controller)
But, last time I checked, Highpoint's website does not list any
compatible cables under their "Accessories" category.
For confirmation, also ask Patrick Kennedy, who has much more experience
with server assemblies than I do; and, above in this thread Patrick has
already mentioned his meetings with Highpoint staff at trade show(s).
Hope this helps.
p.s. If you get your hands on a 3840A, would you please post an update here?
I'm dying to see some empirical measurements of a RAID-0 array with
four Samsung 960 Pro SSDs, assuming the 3840A is installed in a
compatible PCIe 3.0 x16 expansion slot. Intel's model 750 SSD
should also work in this same wiring topology, but the 750 is much
more expensive that the Samsung M.2 models.