This amplifier chassis was originally made for four 4CX250B tubes. One hole has been enlarged for the GS35B tube. The remaining three are used as screened air inlet holes from the pressurized chassis. The circuit board on the rear enclosure wall is a voltage divider for the "HV Okay" signal that's sent to the control circuitry. The home made Teflon sheet air exhaust chimney is visible around the vacuum tube.