I was afraid that might be the case :^)
There is a special washer with a tab that implements the stop, and it may just be installed upside down - I remember that was a common problem with that kit. It may be possible to fix the lack of a stop. If not, you might give it a try with a small input signal, e.g. from a phone or computer output that has the volume turned down and learn where the minimum volume position is - usually straight down. The dual switched controls take some getting used to anyhow. The switched level control was an upgrade kit; the original had two 100K audio-taper potentiometers instead.
There are three input pairs. The original had a single output pair, yours appears to have a second output pair added, perhaps to drive a subwoofer.
