What is the difference and why are there different part numbers for the recoil spring and guide rod assembly used on the PX 45 and the PX 45SD?

PX 4 .45 Recoil Spring

The PX 45 uses a single wire coil spring on the guide rod assembly, part number C89135.
The PX 45SD uses what is referred to as the SOCOM recoil spring guide rod assembly, part number C83730. The SOCOM guide rod incorporates dual wound coil springs. The purpose of this is to ensure positive functioning of the pistol under the most adverse conditions experienced under combat conditions.  While the parts are interchangeable between the two pistols it should be noted that the SOCOM recoil spring guide rod assembly should only be used with typical service/duty load .45 ACP ammunition. 185 grain target ammunition and or other light load specialty ammunition may cause malfunctions in a PX 45/PS 45SD fitted with the heavier poundage SOCOM recoil spring guide rod assembly. In all cases any pistol being used for duty/personal defense should be function tested with the exact ammunition intended to be used to insure reliable operation.