The later version installed a detent and machined hole in the trigger group frame to prevent an action lock; the detent would prevent the hammer from engaging when the trigger was depressed and would prevent an action lock from occurring.
The later version installed a detent and machined hole in the trigger group frame. This was to prevent an action lock. The detent would prevent the hammer from. The detent was engaging. The trigger was depressed. The detent would prevent an action lock from. The detent is occurring.