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Detail of the injector face.

The face of the injector consists of 20 concentric rings, alternating between fuel and oxidizer injection rings; there are 10 of each type. The rings with pairs of holes (doublets) are the fuel rings, while the rings with sets of three holes (each triplet consisting of two larger one one smaller hole) are oxidizer rings.

Picture 2 of 2.

The center hole appears to be threaded (like the F-1 injector center hole); this initially led me to speculate that it was used to retain a throat closure (as on the the F-1), but on later, baffled versions of the injector, an igniter button was located at the injector's center.