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The Extreme Service thumb safety is the result of years of design and testing to develop a thumb safety that is ergonomically comfortable, rugged in construction and offers unique features that solve many of the problems with existing designs. The thumb shelf has an arced shape that is sculpted to meet the "plate, portion of the safety. The shelf is lowered slightly and rotated on the plate to better match up to the human hand. This makes the safety more comfortable in use over extended firing sessions. The positioning of the thumb shelf also helps keep the palm of your hand in deeper, more solid contact with the grip safety, so it's less likely that you'll have a failure to get the grip safety depressed and then keep it depressed, which could render your pistol unfireable at the worst time. The plate is made thicker than most other brands and is configured so that the safety plunger bears fully on the front of the plate, eliminating side thrust loading from being exerted against your plunger tube which can eventually loosen the tube’s mounting, allowing the plunger pin to climb over the safety's plate, potentially rendering the gun unfireable. An ample detent pocket and radius give a positive "click, as you move the safety on and off. The pivot shaft joins the plate with a generous radius which is crucial to prevent stress breakage of the plate/shaft joint as found on other brands. The blocking lug has an extra fitting pad on the rear of the lug allowing the gunsmith to regulate how far down the thumb safety can swing, which keeps the safety's plate from overhanging the edge of the frame. An industry first! The rear edge of the plate is beveled and blended where it meets the back edge of the frame, a feature lacking on other brands. Looks great and a complimentary design to the Harrison Design Extreme Service Slide Stop. On HD453 and HD454 the narrowed thumb shelf has an arced shape that is scuplted to meet the "plate, portion of the safety. The shelf is lowered slightly and rotated on the plate to better match up to the human thumb. This makes the safety more comfortable in use over extended firing sessions.