Here’s what the (cracked) faceplate of the FC1002 Frequency Meter looks like, through polarizing filters that reveal the internal stress.
A circular polarizer screwed on the lens:
A sheet of linear polarizing film held in front of the lens:
For reference, none of the other instrument faceplates on the bench show anything other than uniform gray, with one exception that points directly to the plastic injection point.
I’d say this plate cracked due to unrelieved internal stresses and not anything I did or didn’t do.
The Smell of Molten Projects in the Morning 