I attempted to get the answer on Google already but no luck. On the Interstate website, it says.....
So.........the Tern, such as this one,Interstate Aircraft Company has undertaken to further refine the Arctic Tern as the ultimate STOL aircraft, including the following improvements to the original S-1B2:
- Replace wooden wing spars with aluminum box extrusion for increased capacity and better service life.
- Replace steel lift struts with aluminum for reduced weight and better service life.
- Replace fixed-pitch propeller with constant-speed for better takeoff performance and higher cruise speeds.
- Upgrade instrument panel, instruments, avionics, and electrical system to modern standards.
- Incorporate Dual Seaplane doors for added safety and convenience.