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A scrap of foil between the turbine exhaust manifold inlet and the thrust chamber.

Page 1-10 of the F-1 Rocket Engine Operating Instructions (p. 37 of the PDF itself), "Overall Engine Inspection for Uninstalled Engines During Receiving Inspection," instructs the technician to

Visually verify that aluminum foil tape is installed over space between thrust chamber exhaust manifold and thrust chamber tubes and that white sealant RTV-102 (General Electric), or equivalent, is installed between thrust chamber tubes and external bands.

If this is the referenced aluminum foil tape, it no longer appears to be installed over the entire required area, but it would still be remarkable that some of it is intact after all these years.