Evolution Gun Works Flat Smooth Mainspring Housing WWI 1911s Steel Blue Carbon Steel Blue. You can use the pins and springs from your original mainspring housing or purchase new parts from us.This mainspring housing makes a great replacement for plastic mainspring housings.This is our premium mainspring housing designed and made by EGW. Fully machined from bar stock. This item will fit 1911 Government WWI 1911s it will not fit STI's or short grip frames. It is a flat mainspring housing. Specifications for Evolution Gun Works Flat Smooth Mainspring Housing: Manufacturer: Evolution Gun Works Color: Black Finish: Blue Fabric/Material: Steel Design/Pattern: Flat Gun Make: 1911 Platform Gun Model: 1911 Condition: New Package Contents: Evolution Gun Works Flat Smooth Mainspring Housing What firearm models does the manufacturer say Evolution Gun Works Flat Smooth Mainspring Housing is compatible with? The manufacturer says that Evolution Gun Works Flat Smooth Mainspring Housing is compatible with 1911Evolution Gun Works 11455: Flat Smooth Mainspring Housing WWI 1911s Steel Blue Carbon Steel Blue