Fusion Pro-Series Carbon steel 2011 style Mainspring Housing, Standard for STI 2011 and SVI style pistols, Replaces the Plastic mainspring housing with steel. CNC Machined from bar-Stock, With 25 lpi Checkered back. Black coated to resist corrosion. All CNC machined in the USA,. No MiM, No Cast, ALL Solid CNC Machined Bar-Stock.