This is a small fin sewn into the center sleeve with a loop for securing wooden spreader struts. It was used on high nose angle designs (and the standard Trooper) to prevent the buckling of relatively long spreaders under load. Now, with spreaders made of carbon fiber, it is no longer necessary.
There was always the problem of humidity. I would fit spreaders very carefully in my workroom, but in hot dry places, or damp humid places, the fit could be either too tight or too loose elsewhere in the world. Tight spreaders can be shortened, and loose spreaders can be tightened with little dowels of appropriate diameter inserted across the ends inside the spreader pockets as shown here.