Why does the MacPro cylinder housing have this weird rectangular marking?

[Guest timidhermit]

I am in mid of purchasing an used Mac Pro 2013. The Mac Pro has been well kept (at least what I think) by the original owner. However, I notice something very odd about the MacPro's cylinder housing. There is a thin rectangular area near the top of the housing for which the shining finish is dulled. Please see photos. Note in the photos a piece of yellow felt is covering the top of the cylinder to minimize light reflection in order to see this bizarre mark

 

They are very difficult to capture...

 

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Source: MacRumors