These OEM mainsprings are mainly original springs, and is the same mainspring that is included in PG Service kits. The springs are High power springs and hold max length the manufacturer recommend for the specified model. All measurements are approximate and may vary slightly, but always fit to the model defined. The springs can also fit into other air rifles / pistols than those listed in the specification.
When replacing the spring Progunner recommend changing the piston seal. This is to ensure that the seal is not damaged, which can lead to the spring failure. Be aware that with the new spring, your airgun may exceed the legal limit. If so it may be necessary to cut some of the spring off.