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I have no idea what this is but I have been thinking about it for a while. It must be part of an X-ray setup, e.g. a diffractometer. I think so because of the way the engravings are clustered.

The engraved symbols are all metals that were used as anodes (targets for electrons) to generate X-rays in vacuum tubes. If you bombard a metal target with high-energy electrons, it will generate a spectrum of X-rays, with intensity peaks at certain wavelengths characteristic for that metal. The main peak is called K-alpha.

https://en.wikipedia.org/wiki/Characteristic_X-ray
http://pd.chem.ucl.ac.uk/pdnn/inst1/xrays.htm

Here are the K-alpha (in angstrom) for the engraved symbols:

Ag 0.56
Mo 0.71
...
Cu 1.54
Ni 1.66
Co 1.79
Fe 1.94
...
Cr 2.29

Notice how the values cluster in the same way as the symbol engravings are clustered along the shiny metal slide.

Also, if you send an X-ray beam through a crystal, it will get diffracted following Bragg's Law - basically 'the sine of the angle of diffraction is proportional to the wavelength'.
I.e. the K-alpha peak in Ag and Mo beams will bend a lot less than the one for Cr.

https://en.wikipedia.org/wiki/Bragg%27s_law
http://www.calistry.org/calculate/braggsLaw

So I speculate that the blue 'hood' can slide such that whatever target is inside can be in the sweet spot of the X-ray beam (that has been diffracted towards the shiny metal base), for different kinds of X-ray generating vacuum tubes. But really I'm just guessing and I don't have any practical knowledge in this area, I just like puzzles.

Luc