There is no HPF although it is likely there is one in the Liner-2.
C1 is to prevent RF entering the circuit, it's value is too low to have any significant effect on frequency response. If I remember correctly, the Liner-2 microphone was a low impedance (200 Ohms?) dynamic type so shunting it with 470pf wouldn't change the response within hearing frequencies. Even R1/C3 have a 3dB drop at ~15KHz so they would be equally ineffective. The output filter, assuming no loading from the following stage has a 3dB drop at about 7KHz so still doesn't do much.
I fear this design is more of a "found these bits in my junk box so I used them" one than properly calculated.
Brian.