Universal changes their Bolt
Sometime in the early to mid 1970's Universal redesigned their firing pin and bolt. Up until at least serial number 196,6xx they had used a GI type firing pin that was free floating and held back from the primer of a live round by the cut on the back of the receiver bridge and the design of the rear of the bolt relative to the hammer. Sometime prior to serial number 197,4xx, Universal utilized a bolt that contained the entire firing pin, held back from the front of the bolt by spring tension. This effectively eliminated the issues related to the improper machining of the receiver bridge that had failed to hold the firing pin back until the bolt was locked in place. This is an issue with every commercially manufactured carbine, and some original GI carbines due to heavy use. Universal is the only company that adopted this new bolt design. This was probably the best change Universal implemented. However, they still failed to harden the front and rear of the bolt to GI specifications, causing rapid wear and a shortened life span for their bolt. These issues are not specific to Universal and applied to all of the commercial manufacturers. This is discussed further on the web page devoted to Safety Issues.
