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Detail of the third injector.

Picture 1 of 2.

Note the three closely-spaced holes; the middle hole in this set is an index hole. The F-1 injector is indexed via two pins, one round and one diamond shaped, which fit into two index holes located 180° apart to ensure that the injector is installed in the proper orientation.

Both index holes are round, although they are different sizes; I believe that the diamond-shaped pin has a larger diameter.

This index hole is approximately the same diameter as the other holes, and so I believe it is for the round index pin.