Jack,
Great question...and I should have answered that one before it was asked...knowing it would be asked!
That is 1/16" thick G-10 material that has been Hysoled in place AFTER building 3 stringer structures into the Stab itself. It's even better now since the load center is even disbursed more since I increased the box lines of the Pivot shaft. Thos four screws only held in the AC Ply I used inside the skin when I was setting the G-10 material in place. I'll just let them stay in rather than back them out.
I did the same on the Fuse pivot shaft hosing too. Did not weaken at all...only strengthened. These are not "functional" hatches...they are closed in and sealed for good.
Rex