I noticed these “nacelle” like protrusions on the first stage of Black Arrow while browsing Wikipedia. They look like they're spaced unequally but symmetrically in two sets of two around the first stage.
I couldn't work out their purpose, they look like they could contain some sort of SRMs to aid with staging but they are facing the wrong direction. My next through was that they might be some sort of sensor (pressure, turbulence, etc) but I can’t find any definitive reference.
I also noticed similar protrusions on the Cyclone-4 first stage from a recent Parabolic Arc article:
Do these protrusions share the same purpose, and what is that purpose?