The Sentinel/Wonder Wizard machines Star Explorer, Demolition Derby, C.B. Charlie, and Flying Aces were originally designed for 2" flippers. However, it is believed that their marketing department felt 3" flippers would be better (after all, this was 1977 and all other pinball machine manufacturers were using 3 inch flippers). So 3 inch flippers were rushed into production. These had an offset shaft so in reality they were more like 2 inch ones anyway except they felt awkward.
If you want to convert to 2 inch flippers and improve play, here are the part numbers and quantities needed.
(1) pair of Gottlieb # A11242-3A Flat Top 2" white flipper bats with blue lettering ($12.00 per pair at Pinball Resource)
(2) Gottlieb # A6888E+ Thin Flipper Shoe and Shaft assembly ($8.44 each at Pinball Resource)
(2) Screw to attach flipper bat to shaft, # HDW-632-1-1/4 (27 cents each at Pinball Resource)
(2) # RIN1 Rubber Ring, 1" white (30 cents each at Pinball Resource)
If you want a more modern, cool looking flippers, use either one of these following molded bat & shaft assemblies:
(2) Data East # 515-6275-06 or 515-7191-06 Yellow 2" molded plastic bat & shaft ($8.00 each at Marco Specialties)
(2) Williams # 20-9264-6 Yellow 2" molded bat and shaft ($4.95 each at Marco Specialties)
(2) 1/2" by 1" flat flipper rubber for above molded bats (about 65 cents each)