Dr. Wortmann published a paper in 1973, "Symmetrical Airfoils Optimized for Small Flap Deflection", which included the 71-L-150/25. I don't know the difference, if any, between that and the 71-L-150/K25. I presume that K is for Klappe (flap). He designed these airfoils for surfaces such as vertical and horizontal stabilizers to have minimum drag when the control surface was deflected as for trim. I don't think this is applicable to stunt, but Phil Cartier and I looked into using this airfoil for combat planes. I found my hand plot of the 71-L-150/25 this morning. I think I went so far as to make a flap, but I didn't finish the airplane. As I recall, to break even for induced drag from the added weight, we would have had to build the flap and mechanism for less than 50 grams. That seemed impractical.
The "Symmetrical Airfoils Optimized for Small Flap Deflection" (note American spelling) paper heading had the logo of OSTIV, and at the bottom was "Aero-Revue 3/1973". The 71-L-150/25 and similar airfoils are in Stuttgarter Profilkatalog I, by D. Althaus, Institut für Aerodynamik und Gasdynamik der Universität Stuttgart, 1972.
I think the primary consideration for a stunt airfoil is maximum lift coefficient. That allows wing area to be minimized, so turbulence response is minimized for a given cornering capability. Other considerations are thickness, ease of building, and linearity of the lift-vs.-angle-of-attack curve. The thicker the airfoil, the lighter the wing can be built, and airfoils with high max lift coefficient tend to be thick. Airfoils that are thick and convex just forward of the flap hinge are easier to build and finish than airfoils such as the 71-L-150/25. Igor mentioned linearity of the lift-vs.-angle-of-attack curve and wrote about airfoils with a hump in the curve. I had not noticed such a hump, but looking at the 71-L-150/25 lift-vs.-angle-of-attack curve today, I see that it has that hump at large flap deflections.
I don't think drag is a big consideration for stunt airfoils. Airfoils with the highest maximum lift coefficient tend to have relatively low drag in maneuvers (which is a virtue) and slightly higher drag in level flight than other airfoils. Because we fly the same path every flight, I think we could calculate the energy required for a flight as a function of airplane weight, airfoil section drag, and span loading, and find out for sure how much airfoil section drag matters. I have been too lazy to do that.
Using XFOIL, which I probably don't know how to do, the best stunt airfoil I've found is the Impact's.
Frank's next question will be about the optimal progression of rib spacing from wing root to tip.