Sorry, I should have added that my manual is version 2.2 (3/2006). My plans (revision 2.0) clearly show rabbets on panels #2 and #3, but none on panels #1. I'm going bed assuming it's a manual typo.

-Kristofer 

Nay, there is no rabbet on the Skerry's #1 panels.  The manual speaks with forked tongue.

The #1's are ambidextrous out of the box, but when it comes time to assign "gains" on their upper, outer corners, you need to decide what will be the inside face and what will be the outside face of those panels.

That's what I figured.