This cutaway drawing of the F-1 injector plate has a "throat plug insert" annotated. What is a throat plug? What is it used for?
A "throat plug" is a cover to prevent contamination from entering the engine's combustion chamber from the direction of the nozzle. It can also serve as a "stopper" for leak testing.
I could not find a picture of an F-1 throat plug, but here are some diagrams of the Space Shuttle Main Engine's throat plugs.
This diagram shows the upper throat plug in use during a leak test of the combustion volumes of the engine. (The lower throat plug was used to leak check the combustion chamber / nozzle interface.)
There was also an 'environmental protection' throat plug, provided for contamination protection post-landing.
Source: SSME Pocket Data Book