I should have posted more details, sorry. I checked the airspeed indications on both airplanes with GPS, they are right on. Both are new airplanes (this is in the US). The one with the higher stall speed was weighed several times, the weight seems to be accurate. We have only modified the higher stall speed airplane. The stall speed flaps up and take off flaps is also higher than normal, but not as much as with full flaps (slotted flaps). I measured the angle of attack at which they stall, the one with the higher stall speed also stalls at a lower angle of attack. We checked the flap rigging, and set it to the largest flap deflection within the tolerance, no change in stall speed. I tufted the flap upper surface (low wing), the flow on the flaps was attached at full flaps before the wing leading edge modification, afterwards it was not, even though the stall speed was lower (another mystery, normally this would be a loss of lift). The wing contour is now as close to type design as we could get it, although on one side it semms to be slightly thinner than it should be. Stalls are straight. I'm out of ideas.