hammer spring (mainspring) for 1873, 1892 and 1894 (pre 64) Winchester. This is the spring that puts tension on the hammer. These springs are CNC machined out of high-quality spring steel to original factory specs. The mainspring Screw wp47 us used on the 1892 and 1894, and model 53. wp621-a is the mainspring screw 1873 model.