Two thoughts on this: 1) It is common, on free-flight models (especially rubber-powered), to make trim changes using lengths of 1/16" or 3/32" sq. balsa strip stuck above or below the TE of the relevant surface: this is simpler and neater than messing about with things like card or aluminium trim-tabs, and we all call it a Gurney Flap. 2) Many years ago, when it was normal on C/L stunters for flaps and elevators to be tapered to a knife-edge at the TE, somebody found that a thick, square TE section gave a sharper and more positive control response, especially around neutral; since when, I note, the control surfaces of many C/L aerobatic models remain untapered. The arrangement shown above, with Gurney Flaps top and bottom on the elevators, therefore seems to be just an extension of the 'thick, square TE' principle: the next time I fly an old Peacemaker of mine (with the original, small-area tailplane), I'll add an 1/8" sq. Gurney Flap to the elevator, top and bottom, and see what difference I can detect.