In the process of bringing a Hickok 600a tester back to respectability, a question for the knowledgable.
When I remove the old covering from the cabinet there is an 85x hand written in black on the top case and on the bottom case and on the plug wire bracket. This number does not relate to the serial number so what would it be??
I know....slow night in repairville!

I can only guess that it's an assembly mark. Such at this 85X lid goes with 85X case. All the holes line up, so it fits good. ( yeah, slow there tonight too )