The "A" parts are always side keels of some sort for the fuselage.
here i have a photo of the A8/A9 side keel. if you look closely at the instructions just near where you are supposed to build the fuselage on the plan they show the details for the side keels. this plane is special because it has two side keels on each side. one is 3/32". the other, you will find that it is the one that goes towards the rear of the fuselage, is only 1/16" thick. the idea is that you glue them together as per the plans (you really do need to read the instructions carefully, as the answer is on them already most of the time) and then you are supposed to sand the 3/32 one down gently so the thickness of it all blends smoothly to the 1/16". this creates a taper you will notice. you can see that in my picture. hopefully this helps.
I believe the upper keel is the same way.