Many of the instruments of that era had this removable filter plate, like the 5370A/B, and older ones, but also DMMs, if I remember correctly.
Maybe it is from the time when the display was made of Nixie Tubes and light bulbs for the units tell-tale, so that these could easily be changed when failing.
The filter plate of my 5370B was broken, maybe that was happening so often, due to the size, that they made it easily removable.
If you read the 5370 manual, obviously the LED seven segment displays, and the indicator LEDs were failing sometimes, and they were assembled in sockets.