In the following images, it can be seen that the interstage (separating the first stage PS1 and the second stage PS2) of the Indian Space Research Organisation's (ISRO) Polar Satellite Launch Vehicle (PSLV) contains pairs of cylinders as indicated by the yellow and red rectangles respectively:
Original Image Source: PSLV-C47 Gallery
What is the purpose of these cylinders? Are they separation motors? Or, are they present for passive stability? Why are they flat at the top, will they not increase drag during the atmospheric regime?
If possible, kindly support your answer with references.
Thank you in advance