Tremolo Spring are as vital as the saddle and they are responsible for the fat sound of the 50's and 60's Stratocaster. Most newly manufactured springs have completely different characteristics from their vintage counterparts.
They researched vintage 50s and 60s era guitars and were able to accurately match the tension of the springs.
With the prerequisite of using 4 or 5 springs together, they were able to achieve superb torque feel and stabilized arming that is made possible from the difference in weight and contraction percentage.
With the increased weight they were able to create those fat tones heard in 50s and 60s guitars.
These can be used as a replacement parts for new reissued guitars, enabling their owners to enjoy the characteristic closer to the original.