Left Elevator question

I?m building the left elevator. See section "PREPARING THE LEFT ELEVATOR" page 6-10 in the construction manual. There is one item which says Machine countersink for the rivets that attach E-705 to E-606PP, the flush head can go on either the forward face of E-705 or aft face of E-606PP.

I don?t understand why there should AN426 rivets be used and what confuses me even more is that the flush head can go on either side. So how did you do it and why? Here is a pic.

There are a number of places that an AN470AD3 rivet is appropriate. However, the RV7 kit does not include any. Ever. I imagine that same is true of the other kits.
The locations are few enough that it is probably an inventory decision.

If you wish, order some AN470AD3-3.5 or 4 and then some longer ones like -9 so you can cut to fit any situation.

Otherwise, finish per plans.

I believe it's because you don't have any -3 universal head rivets in the kit.

These answers making sense. There are no AN470AD-3 rivets in the kit. So actually there is no need for the flush rivets in this location. I have a selection of AN470AD-3 rivets, so I will use these.
Thanks for the replies.