Hardware Reference Manual (1st Edition 1986): Page 136: Channels 0,3 left and Channels 1,2 right.
Hardware Reference Manual (3rd Edition 1991): Page 137: Channels 1,2 left and Channels 0,3 right.
Looks like whomever wrote the later manual (be it starting from 2nd or 3rd edition) swapped them around for some reason given that the details are in the same respective sections of the manuals. I do not have the 2nd edition to check that.
Here's what the 2nd edition says:
Hardware Reference Manual (2nd Edition 1989): Page 134: Channels 0,3 left and Channels 1,2 right.
In fact, here's the whole paragraph:
DECIDING WHICH CHANNEL TO USEThe Amiga has four audio channels. Channels 0 and 3 are connected to the left-side stereo output jack. Channels 1 and 2 are connected to the right-side output jack. Select a channel on the side from which the output is to appear.
