This service bulletin came out last fall and calls for annual inspection of the rivets at the bottom wing skin and to add this doubler if any signs of wear. I elected to just install the doubler and not wait. Note that the SB still calls out annual inspection for wear, but this makes it extremely unlikely to find such. There are no less than 4 separate rivet types in this doubler. The zig-zag row on top and the lateral portion of the middle row are ordinary LP4-3’s. The next 8 in the middle row are 5/32”, and the inner most rivet (still on the middle row) is a Cherry Max. The bottom row of zig-zag are very low profile (inner surface) since there is very little clearance below them. All in all, it was a nice day of good old fashioned metal work.
wing root doubler