The hole above the filament rheostat (the right hand knob) was there so that the operator can see how bright the filament of the tube was. This radio was used with a UX200 tube, which ran fairly bright. The peep-hole allowed the operator to visually inspect the brightness of the tube while it is operating. This hole was removed on the later model 50, as the UV/UX201 tube was more common and burned at a lower brightness.