JHS -- The front panel lamp array signifies channel operation as designated by the Stereo/Mono mode selector switch. A and B stand for channel A and B inputs, while L and R stand for Left and Right outputs. The center red (C) lamp is normally dim to act as a power indicator, and goes bright when the powered center channel speaker output is activate to any of its selectable level settings other than "off".
40 watts is a Music Power rating which Fisher used extensively when that rating mode came into vogue. Today, with healthy tubes, real world RMS power output is about 35-36 watts RMS maximum from each channel when driven individually, and about 30 watts RMS with both channels driven. With multiple output tap output transformers employed, this rating is maintained whether the output impedance is 4, 8, or 16 ohms, as long as the load is connected to the proper output taps.
I hope this helps!
Dave