Hardened, tool steel bar is used as an anvil inside the frame rails on a 1911. To use, measure the rail thickness on the slide. Then, insert the fitting bar with the same thickness into the frame rails and tap the frame rail gap until it is the correct width to accept the slide. 1" wide x 4" long. Number listed is the thickness of the bar. For use, measure the slide rail thickness, place slide fitting bar (use closest thickness to measurement) in frame, then press or tap frame rail to close the space to match the slide rail thickness.