It is a clever hidden placket, in fact. I may have to make this a third time just to try that cool hidden placket.
You can see in the pattern instructions (picture at left) that the outside vertical band goes all the way up to the neck edge, whereas the inside vertical band (really the fly for the placket) stops at the collar's lower edge.
So, the outside collar piece has to be shorter than the inside collar piece, Now I get it. You probably got it a long time ago, if you have worked with this pattern.
And...the error I mentioned in the lining for the front of B...totally operator error. Oh, my.
So, so sorry for documenting errors that are not there!
Bottom line - this is a GREAT pattern. Now go buy it, if you haven't already.