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A grouping of F-1 rocket engine stud protectors. These styrofoam pucks would be mounted on the engine's studs on which the thermal insulation would be mounted to protect against injury during shipment and handling.

Officially, these are "RK-395-10100 PROTECTORS, Stud, thrust chamber" and one is shown as item #35 on the F-1 engine closures diagram on my F-1 Engine Thrust Chamber Throat Security Closure page. There would be 119 installed on an engine. The nozzle extension, which also mounted thermal insulation, was fitted out with nutplates, rather than studs, to secure the thermal insulation, and presumably did not require stud protectors.

The wider end would be installed facing the engine. These can be seen in various vintage F-1 rocket engine photos.

The markings on the third stud protector read "RK-395-10100" (its part number) and "10001-1555459 N.C.".

The fourth stud protector simply reads read "RK-395-10100".