The S-IVB, the second stage of the Saturn IB rocket, was the one of the largest liquid hydrogen stages produced in the 1960s. Equipped with one J-2 rocket engine, it produced about 200,000 pounds of thrust. It was 21 feet 8 inches in diameter and 58 feet 5 inches high.

Structurally, the S-IVB stage consisted of

• An aft interstage, which mated the stage to the S-II second stage. Although I tend to associate the interstages with the lower stage, the interstage was properly part of the S-IVB.
• A cylindrical aft skirt, to which was connected the conical thrust structure. The engine was mounted to the thrust structure, which transmitted the thrust to the aft skirt, which in turn transmitted the thrust to the propellant tank walls.
• A liquid oxygen tank and a liquid hydrogen tank. Rather than each tank having customary hemispherical bulkheads at both ends (and separated by an intertank structure, as with the S-IC), the forward end of the LOX tank and the aft end of the LH₂ tank were separated by a common bulkhead. The common bulkhead consisted of two aluminum domes forming the ends of the respective propellant containers separated by honeycomb insulation; the honeycomb insulation was necessary to prevent the LH₂'s -423° temperature from freezing the liquid oxygen.
• A cylindrical forward skirt, attached to the forward end of the LH₂ tank, which transmitted the thrust loads to the Apollo payload. Range safety and telemetry antennas were mounted on the exterior of the forward skirt, and the interior of the forward skirt had cold plates to provide environmental conditioning for various electronic equipment also located on the walls of the forward skirt.

Click image for a 5351x4117 pixel version of this image in a new window.
From page 16 of the Saturn S-IVB Training Graphics: Systems Familiarization, Saturn V (Revised October 1967), located in the Mauldin collection at the Dept. of Archives/Special Collections, M. Louis Salmon Library, University of Alabama in Huntsville.
Scan and reconstruction by heroicrelics.org.

This cut-away diagram shows the major structural elements assembled into the final stage.

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From page 5-1 of the Saturn S-IVB Flight Manual (MSFC-MAN-205), located in the Mauldin collection at the Dept. of Archives/Special Collections, M. Louis Salmon Library, University of Alabama in Huntsville.
Scan by heroicrelics.org.

This diagram calls out a number of components and features:

Dated September 1968, the following diagram shows the S-IVB configuration which launched Apollo 7:

Click image for a 1433x1946 pixel version of this image in a new window.
Taken from the page 4-4 (p. 66 in the PDF) of S-IVB-205 Stage Flight Test Plan.
Extraction and cleanup by heroicrelics.

This diagram calls out a number of components and features: