I've run across a few of these, they have a frequency response that has a little bit of ripple in it and a slope, presumably to counteract a system with the opposite ripple and slope.  The plot shows the frequency response for this particular unit - I suspect that whatever it was equalizing worked below 15 GHz, the large swings above that don't seem like they are what was planned.  Adjustable via the slugs on the side that has the label and also the bottom which has a cover held on by 6 screws.

  The 674671 is the CAGE for Aeroflex/Inmet