.400 spine is 55/75 and .500 spine is 35/55 (although every single GT shaft I have put on a spine tester shows stiffer than what the shaft is labeled).
Unless they changed on the newest ones for 2016, both numbers should be on the shaft.
The modern manufacturer's deflection ratings are done on the ASTM chart, and derived by checking the deflection on a 28" center with a 1.94# weight.
When spine is checked using the AMO standard (26" center with 2# weight) it will always show a higher spine than the ASTM.
EXAMPLE:
I have some shafts that ASTM spine as .300, and they are dead on.
When checking their spine on a spine tester using the AMO formula, they have a .250 deflection.
To see what the ASTM charts actually are in AMO spine, you can use the conversion formula of ASTM deflection X .825.
Example:
ASTM spine defletion of .300 X .825 = AMO spine deflection of .247
Shafts from different manufacturers, and different materials will vary some, so the conversion formula is not perfect, but it will get you close enough to make accurate shaft selections when viewing the manufacturers spine charts.
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