RE: Meter question on older Futaba FG series...
According to the instruction manual that came with those transmitters:
The "Square level meter indicates the transmitter output power and indirectly shows power supply voltage."
In other words, it's an ammeter that is showing the current flowing through the final output stage of the transmitter, and when the meter starts to go down, the battery pack voltage is going down. That's because voltage is the "push" that moves current. When current declines, it indicates that the voltage is declining.