How to test a slide potentiometer
I'd say that you are correct that the pot is faulty. No other component failure would give 'OL' readings on the meter at your test points.

The circuit is a little strange. Neither end of the pot is directly grounded on this board. For a standard volume control it would be most unusual to have resistors or capacitors in the ground leg of the potentiometer. This makes me suspect that R+ and L+ are the actual grounds. You should be able to test this by pluging back together and checking for continuity between the points on the board and signal ground - maybe on the audio out connector.

I can't help with part number diagnostics. I gave up on tracing Japanese parts in the pre-Internet days. Maybe it's easier now.