3 different era Sigs 3 different main spring and struts.

Remember, the early pinned designs were originally made, when "user" maintenance was basically field stripping for cleaning, and any further was for trained personnel. Any further disassembly was performed by factory trained personnel (armorers) or knowledgeable gunsmiths. Anyone who has swapped Mainsprings on the older pinned assemblies, have always needed more than 2 hands, as they are tiresome. While they can be smoothed up, there is a lot of possible metal to metal contact, which can lead to friction. The plastic Seat does lessen metal to metal contact with the Seat, but still allows contact of the Strut with the Mainsprings coils.

While doubtful, the heel magazine catch designs will work any other way, than they currently are, the side magazine catch allowed "change".

The pinned assembly allowed a longer Mainspring, giving a reasonable mechanical advantage. When changed to the "New Style" for easier Mainspring changes, the Mainspring was mounted higher and raised the base, making for a reduced mechanical advantage, so the improvement to the "E2" style strut, moved the Mainspring lower by lengthening the strut, regaining the mechanical advantage.