It looks to me like the whitish material may be the bottom of the sample container, that the sample is not very deep (perhaps one layer of grainy/dusty material) and is nonexistent in a few areas. In black and white,* the whitish material looks similar to the nuts that are visible.
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wayne asked: “I have an ideal how big this actually is, but how big is this?”

From my extremely well calibrated eye and years of designing somewhat similar scientific instruments, I suspect that the empty threads are for number 4 screws. Maybe number 6, but probably not number 2. Number 4 is 0.112″ outside diameter, making the ring containing the sample about 1 inch (2.5 cm) in width, so the outside diameter is somewhere around 12″ (33-ish cm) and the inside diameter around 10″ (25-ish cm).

“If I’m recalling this correctly- didn’t they have to drill through some screws to open this up? Does this image show the aftereffects of any of that work?”

I don’t see any scratches, shavings, burrs, or anything that suggests they had any trouble taking it apart. The later reports seemed to suggest that they didn’t drill out the screws but made a tool to remove the screws in a way that prevented contamination of the sample.

“‘It’s Growing….’”

wayne,
did you see that flicker in that grain in the upper right of the picture? You didn’t change the lighting, did you?

Oh, that was me! That was me. I changed the brightness on my computer. False alarm. Everyone calm down while my heart rate recovers.
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* Although the center looks a bit like sepia, so maybe it isn’t as black and white as I think it is.

If I’m recalling this correctly- didn’t they have to drill through some screws to open this up?
Does this image show the aftereffects of any of that work?

The Andromeda Strain (1971)
“It’s Growing….”
https://youtu.be/9Irf9F2RB2M
(2:52)