Old schematic sign "OVEN"
If you look at the PCB layout in the 179-140 instruction manual for compressor amplifier 179-130 on page 9 you'll see R56 close by Q1 like this: -



And I suspect that Q1 has a heater coil around it that may be called "OVEN". Q1 is a JFET that is used to control the amplitude of the signal (hence form a compressor) and although the heating side of things isn't totally clear, I believe that it is warmed either to enhance the compression circuit (deeper compression) or to keep the circuit stable in different ambient temperatures. Q1 and "OVEN" connection (page 8 of the linked document): -