Extreme Service Mainspring Housings are one of the few parts in modern day 1911s that will readily "drop-in" many frames. However, due to the over 20 manufacturers of 1911s worldwide having different standards for dimensional tolerance cotrol, sometimes you find a combination of frame and mainspring housing that need some fitting to fit together smoothly. It is also possible to find combinations that have slight mis-alignments of the pin hole between the frame and MSH. Both are easy fixes well within the skill of the DIYer.
Extreme Service Checkered Mainspring Housings for Government and Commander size frames
Extreme Service Checkered Mainspring Housings for Officers ACP, Compact and other reduced length frames