There's a lot going on on the back of a 2542.
Orange Box: Cassette Assembly covered by its aluminum cover.
Yellow Box: Cassette Power Supply
Red Box: Low-Voltage transformer for the entire Master Station.
Blue Box: Clock Assembly
Pink Box: Amplifier Power Transistor
Amber Box: FM Tuner Assembly
Green Box: AM Tuner - Power Supply - intercom Circuits
Light Blue Box: Voice Message Controls
Red Arrow: Main Power Cord. The 2542 has a 110 volt power connection in the wall housing, much like earlier vacuum tube designs. This might have been done because the clock requires 110 volts (remember it's a oven clock)