RE: 12z/14mz HD gimble springs
What it sounds like is that you have one of the newer 14MZ Airplane version transmitters that have the Futaba Heavy duty springs already installed on it. If you compared the two and they are identical, then yours already had the heavy springs. Older versions of the radio and a spring with the part number HSP78961.
The HSP79241 is the only heavy spring that Futaba recommends for the 14MZ and 12Z due to heavier springs causing damage to the stick assemblies.
Sincerely,
Krysta
Product Development & Support Specialist
Futaba Service Center USA