ISS/Howard Markman HM-1 Headphone monitor switch box.

     You are looking at an ISS/Howard Markman HM-1 Headphone Monitor box.  I made up a few of these a while back, 1 of which was for my friend, who is mostly deaf in 1 ear.  He is a musician, and it is very difficult for him to tell the stereo content of his mixes while using headphones. 
     This box takes a stereo headphone signal (you can use either 1/4" or 1/8" connectors), and you can select one of 4 output conditions (also on either 1/4" or 1/8" connectors - they are paralleled).  The first setting is 'M' or mono.  The 2 sides are resistively summed together, giving a true mono signal.  The second setting is 'R' or Right.  The right input is sent to both sides.  The third setting is 'L' or Left.  The left input is sent to both sides.  By switching between the 'R' and 'L' and 'M' settings, my friend can effectively discern how things are panned and set in a stereo setup, without actually being able to hear it.  The last setting is 'ST' or Stereo - normal operation for people who can hear normally.  The box does this passively, no power or batteries needed.  The summing resistors lose 2 or 3 db at most of level, which is easily made up with your headphone out.  So, if you can only hear properly in 1 ear, or you know someone else who does, this box is for you!