The Atwater Kent Model 12 (#4910) is an interesting radio in that it has a switchable audio output stage so the user could select higher volume or longer battery life. In one position of the switch, the audio section uses two tubes connected through an interstage transformer. The second tube is the output, driving a high-impedance speaker directly. In the other switch position, the first audio stage drives the speaker directly. The second stage and the interstage transformer are disconnected, including turning off the filament.
The switch can be seen at the front of the right-hand tower in picture #6.