Black squares on gauges

[RAFAIR100]

On some of my older blind flying panel instruments, I get black rectangles which spasmodically appear and disappear on the faces of the gauges.     Does anyone have any idea why?

[H]

I'd guess that there was an added readout box for those gauges. They are, perhaps, add-ons? If so, they may have been edits of a source gauge. For instance, a clock my have been edited for use in a particular panel and does not implement the 'frame rate' readout.

8-)