Unknown brand, commercially made low pass filter. It looks exactly like a B&W filter, so maybe it is. This would be for up to 30 MHz use. The photo of the HP8754A network analyzer has 50 MHz exactly in the middle; 10 MHz per horizontal division, 10 dB per vertical division. Note that at 50 MHz attenuation is 46 dB. At 60 MHz attenuation is 64 dB