No wonder the Soviets had to use the "pear" to adjust the nozzle cross-section.
It's unusual, though, that both the booster and sustainer motor curves are relatively flat. The former apparently has fourteen exposed rods with single circular channels (easy to see how a roughly constant area of combustion is maintained), while the latter is quite unusual.
My interpretation is that the length of the grain containing the "cut-outs" (#2) has a large initial burning area, to compensate for the fact that the central channel has yet to expand much right at the start of operation. During the burn, the area of that part declines, while the area of the channel increases, roughly maintaining flat thrust. If so, that's a fairly clever solution, and one which likely was less expensive to manufacture compared to the typical "star-shaped" channel associated with flat thrust in textbooks.
