RA-47M H1 and H2 Header Explanation:

Headers H1 and H2 are the interface for installation of an optional audio filter.  
While this header can be used for multiple functions, its main purpose is for easy interfacing 
of the FL-10 filter board.  When the FL-10 is not installed, a simple 2-pin header jumper is 
fitted onto the center two pins of the header completing the audio path between the CM119 and 
the buffer audio amplifiers.  

To use the optional FL-10 filters, simply remove the 2-pin jumper and install the FL-10 
observing polarity.  The ground connection is nearest the silkscreen markings "H1" and "H2".