The rear screw is likely an adjustment screw for the mainspring. If you wind it in, it will lift the spring and thus let the hammer trigger drop, perhaps allowing you to cock the rifle with the trigger unset. There is a fine line between when everything is perfect, and then nothing works.
If you wind it out, the mainspring will put more force on the back trigger, forcing its arm higher, and interfering more with the lock sear. These adjustments usually need to be done with the trigger guard removed. Otherwise, it is difficult to get at the head of the screw, and you'll certainly bugger it up trying to get a screw driver in there.