There are 2 types. They are identified by a colour code painted on the spring. Both have the same specs. but different part numbers.
Type 1: yellow-red or violet-red
Type 2: violet-yellow-orange or yellow-yellow-orange
The free length for both is 49mm.
You then must load them to a length of 30.4mm and determine the force necessary to do this. The specs say 1084 Newtons new and are considered unservicable if the force is less than 760N. Precisely how you would do this is up to your ingenuity.