Memory fails me but I believe Handley Page had a machine with a slot built into the section - anyone recall?
According to
Torenbeek/Wittenberg, your memory serves you right: "The development of wing flaps began in 1919 when the Englishman F. Handley Page invented the slotted slat at the wing leading edge which postpones stalling to a higher angle of attack." (
Torenbeek/Wittenberg, Flight Physics, Dordrecht, 2009)