Many Zenith "Black Dial" radios are back lighted with 2 or more dial lamps. Originally there was a protective diffuser shield over the lamp to spread the light more evenly and to protect the dial scale from being overheated by the lamps.
Over the years, most of the diffusers have deteriorated to the point they no longer serve their purpose or crumbled and gone altogether. Often they and the dial scale were damaged by using dial lamps of a higher current than intended.
It is very important that these (diffusers) be in place and the correct lamps be used especially when replacing the dial scale with a reproduction.
I offer a free pattern so you can make your own or I'll make 'em for you.