This is a combination sloped-gain filter and CM isolation PCB.  It is designed to improve available dynamic range for SDRs  over the MF-HF range by attenuating MW and low HF signals while providing minimum insertion loss at 30 MHz.  The idea is to use this with a system which has enough gain at 30 MHz to reach the propagated noise floor per ITU while protecting an SDR from both unwanted CM currents and the very large signals often encountered at lower frequencies.  Because the slope of the attenuation is less than the ~27 dB/decade of propagated noise, adding MF attenuation doesn't risk compromising MW SNR.

This plot is if one had ideal components.  In reality the stop band will only be down about 16 dB because of the series resistance of the indicated JLCPCB Inductor.