White's has used the 6.592 kHz frequency for decades, as have many earlier Bounty Hunter and Teknetic's models. But w/o an engineer's technical explanation, let me just say the manner in which the circuitry was designed was different. A most recent example is their Classic series. Before the Classic SL line they had the Classic I, II and III which operated at 6.59 kHz and shared the same search coil set. It was the same as the former upper-end models.
Then White's made a change in 1988 when they brought out the Eagle II, 6000 Di Pro Plus and 5900 Di Pro Plus and introduced their new Blue Max series of coils. The standard coil on these three models was the 950. But it wasn't compatible with the former models, nor were the former coils compatible with these models.
While it might be possible to connect a mis-matched coil to a detector, and even notice some level of performance, it is not going to be fully compatible and provide the best all-around performance in all modes or settings.
Monte