Hey Msilvestro. I am an A&P mechanic and was in business to cover real aircraft. When I retired I had recovered 35 planes. Seven of them were J3 Cubs. I have never seen false ribs that were ribbed stiched. The rib stich spacing or at ever three inches except for the ribs in the prop blast where they are two inches. On aircraft with more horsepower the ribs stich spacing is different. I have rebuilt a Stearman, a Staggerwing beech and covered an EAA Acrosport. Hope this helps you BOB Moore