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In the image below, we see some bumps at the conical section of what I believe is the transition from the first to the second stage.
What function do they serve?
Image credit: Wikimedia Commons
(Cropped from here)
They are retro-rockets used in the stage separation system.
A CFD image of the thrusters firing:
Supplemental to Organic Marble's answer, this is what the interstage looks like underneath the covers:
(found somewhere on the CapcomEspace website, but I can't find the exact page any more)
Required, but never shown