The BBC's Skylab: The myth of the mutiny in space shows the image below of one big circular end bulkhead of the main Skylab main space, with an astronaut in the center hatch and giant air manifolds and other things attached to it.
I see four cylindrical devices that look like they are roughly 12 cm in diameter and 70 cn long. They have what looks like metal caps at each end and about half of the length is metal and the other half appears to me to be transparent (e.g. a glass or plastic tube) because it show almost the same color as the surface behind it, except brighter. That increased brightness and possibly other visual cues make me feel as though the transparent section is filled with a liquid.
The transparent section is divided into four sections with what looks like three black lines drawn at the "25%, 50% and 75%" points.
The whole structure looks to me a lot like a giant spirit level.
Question: Did Skylab have several giant spirit levels? That's what these look like to me, except there's no bubble.
above: detail from the full image, cropped and rotated, below example of a spirit level. Source