Ok, when you get it, turn the volume pot all the way down, looking at the shaft that is counter clockwise. Measure from the center pin of the new Alps pot to one then the other of the pins. The one that is shorted, zero ohms to the center lug, is the ground. That is like the right lug (as seen on page 22 of the manual) grounded to the input RCA plugs outer connector.
If you and I are right in assuming the center lug will be the "wiper" those lugs go to terminal 1 and 5 like on page 23, second picture, of the manual.
Then that third lug goes to the selector switch's center terminals, one to the left side one to the right side.
Does this make better sense?