This is a sort of bandwidth filter, centered on the mid-range frequencies.
The bandwidth allowed by the filters centers at 517 Hz (the red line in the top Bode diagram), which is certainly mid-range. In the original Bluesmaker, the Low-Pass resistor was 1 M, and the capacitor was 150pF. In this amp the low filter is 2M/500p.
This change shifts the low-pass 3dB point from the original 1.06 kHz to the 159 Hz of this amp. It also shifts the BPF center frequency of the Cameron amp to 517 Hz from the original Bluesmaker's center of 1.33 kHz (orange line), which is more of a true "High-Mid" boost.
Note that the switch in "Off" position puts a series resistor of 3.3 M into the filter ground. This does not "turn off" the High-Mid Boost, it shifts the 3 dB point of the High-Pass filter from 1.68 kHz (On) to 97 Hz (Off); rendering the effect unnoticeable, even though it is still working. Why not eliminate the 3.3M resistor and have the circuit be open when "Off"? The 3.3 M resistor allows the capacitors to charge when the switch is open, preventing switch "pop" when the switch is closed.
The unique McIntyre High Mid switch acts as a low-pass filter connected to a high-pass filter. The low-pass filter cutoff is calculated to be 159 Hz, and the high-pass cutoff is 1.68 kHz. As McIntyre mentions in the Guitar Player article, the High-Mid Boost works independently of the Treble control, as it takes the unfiltered signal from the cathode follower, and then inserts it after the tone stack.
The Resonant, or Center frequency is:
fr = √(fL * fH)
fr is the resonant frequency
fL is the Low 3 dB point
fH is the High 3 dB point