Rear spar mod?

Looking at some pa 18 wings,
what are these doublers riveted to rear spar cap over lift strut attach?
both wings

Looks like the wip gross weight increase. Is it a certified cub? The fuselage should have a small tube going from the door frame to the rear seat tube as well. And square tube at the top
Of the landing gear front to back


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RaisedByWolves is correct. It is the Wip 2000LB up gross. I installed this mod two years ago in my cub so am familiar with it. The piece cherry maxed to the rear spar is exactly like the one I got from Wip. Included in the kit (actually has a bunch of parts and pieces) is also the specific gussets used to attached the ribs to the cap strip clearly shown in the picture. So yep, Wip up gross kit.