In that case there is no way to get it wrong
If your using velcro to close the back that should help with the misalignment. If not, then you will have to off-set the parts at the front so it lines up at the back, or vice versa. Just remember the joining strip on the back goes on the outer half.
Just an FYI, the AP did used to have three seperate parts with only one part doubled up. Looking back it was the inner left that was missing.