HP-48's display with an unusual(?) stain

[vaklaff]

Hello

I got hold of a 48 with a stain. Its nature is new to me:
display on
display off
The look of it changes from black to light grey (even a bit glossy), depending on the viewing angle and light.

It doesn't look to me like a case of the "display bleed" (that would be more compact and always black) nor the usual "polariser rot" (that would be circular and placed in the middle). I suspect it's dirt or something directly on the LCD's surface or in the layers above. More likely the latter, i think I can see a shadow cast by the stain like there's some depth beneath it. But then how could that something get there? The layers are pressed together, aren't they? There are no traces of the calculator ever being opened.

Has anyone experienced that? Any advice how to deal with it?

TIA
v.

Edit: Grammar