Vintage Synth Repair

generally speaking, "audio" in the context of a variable resistor
means "logarithmic taper". "rev" is an additional indication that the
taper should be reversed.
there's no harm in substituting a standard log taper pot or a linear
pot of the same value in such circuits, but the behaviour of the pot
once one starts to use it will be incorrect.

if you attach a resistance meter to a normal "audio taper" pot & watch
the value as you move the pot, you'll see that all the action seems to
be piled up at one end of the travel of the pot. 

now, in the context of adjusting the feedback of a filter circuit, you
can see why you'd want the low-end of the pot's travel (working from
fully ccw, or the bottom of a slider's travel) to make a lot of
difference, & for the control to be of a higher resolution towards the
critical part where the circuit begins to self-oscillate.

with a linear control, the fine-control as the filter goes into
oscillation isn't so good. with a regular log pot, the control is
almost unusable.

hth-
duncan.