Like most engines of its time, the F-1 engine's thrust chamber is comprised of regenerative cooling tubes. As one might expect from rocket scientists, each of these tubes is identified by a number; certain tubes on the F-1 are marked in a way so that those of us who are not rocket scientists can identify them as well.

As described in the F-1 Engine Familiarization Training Manual (R-3896-1) [direct link to 16.8M PDF file at UAH's USSRC Archive]

The thrust chamber body is a furnace-brazed, tubular-walled, regeneratively fuel-cooled, bell-shaped chamber incorporating two outrigger arms to support the turbopump and two outrigger arms to which the gimbal actuators attach. A fuel inlet manifold and a turbine exhaust manifold are welded to opposite ends of the chamber. One hundred seventy-eight primary tubes, hydraulically formed from 1-3/32 inch outside diameter Inconel-X tubing, make up the chamber body above the 3:l expansion ratio plane (approximately 30 inches below the throat centerline plane). Three hundred fifty-six one-inch-outside-diameter secondary tubes of the same material form the chamber from the 3: 1 to the 10:1 expansion ratio plane. A raised weld bead with the tube number and a directional flow arrow, identify fuel-up tube No. 1 and fuel-down tubes No. 60 and 120 on the chamber internal faces of the injector end ring and fuel return manifold. External to the chamber the same tubes are similarly identified on reinforcing bands and straps below the thrust chamber throat.

Two secondary tubes are brazed to each primary tube at the 3:l expansion ratio area plane. Every other primary tube is a fuel-down tube and is slotted on its outboard side at the fuel inlet manifold area into which fuel from the inlet manifold is directed. An orificed plug is brazed into the tube above the slot to permit 30 percent of the fuel to go directly to the fuel injector manifold. The remaining 70 percent of the fuel is used for regeneratively cooling the thrust chamber and is directed down the tube to the fuel return manifold at the end of the chamber. From the fuel return manifold, the fuel is directed by the adjacent fuel return tubes to the fuel injector manifold. The return manifold is welded to the bottom of the thrust chamber secondary tubes and incorporates four drain ports, located 90 degrees apart, to drain residual fluids. Forty lugs are welded to the inside wall of the return manifold for attaching the turbine exhaust leak-test fixture.

I first became aware of the markings on the F-1 tubes while processing my photos of F-114-2 at Stafford Air & Space:

Click image for more information about this picture; opens in a new window.
Picture by heroicrelics.org.

Since learning about the tube markings, I've tried to locate the tubes on the F-1 engines I've visited. The markings on the fuel return manifold (i.e., the aft end of the engine proper) are easy enough to spot, but in order to help me locate the ones near the thrust chamber throat, I made this handy reference diagram:

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Unfortunately, I hastily threw this diagram together while preparing for a trip, and I don't remember the original document from which I captured this diagram.
Capture and markup by heroicrelics.org.