The screen is supposed to be removed when the radio is installed.

Here is a web directory of the 54/55 radio parts in a NOS set.

Here is the instruction sheet.

I tried to enhance the detail in this part of the sheet. Check out step 3:

3. Radio Receiver: Remove the radio grill screen and discard. Tabs should be taken to leave tabs in the forward position as shown in figure 1.

This information caused me to look again at the reprint 1954 ACCESSORIES INSTALLATION MANUAL - page 92, step 7. and illustration 213.

Mystery solved - the screen is standard equipment that is supposed to be removed when a GM original accessory radio is installed.

(Thanks, Greg)