I have an old Kenwood TR-1930 sitting around. I think that my TR-751 (used to drive
my high band transverters) has a problem with SSB receive audio been low. I am thinking
that maybe the TR-9130 might used to compare the audio levels and if needed pressed
into service.

http://www.kb2ljj.com/data/kenwood/tr-9130.htm

The link above has some interesting notes. I have take a couple of screen captures
for future reference in case that site goes away over time.