Top tang screw...probably called other things to other people but that's a common term. Go ahead and tighten it as it should be firmly tight as not to loosen up when the gun is fired. It usually screws into the trigger plate (into the lug that the triggers pivot on) and is a primary securing force of the action to the wood in a boxlock. That lug on the trigger plate usually has a slanted surface on it's forward face that this screw will pull upwards against the stock wood in a properly inletted stock and force the action and stock to seat firmly together. You can see that in the Matador schematic. In an improperly bedded stock, sometimes tightening the screw to the necessary point it needs to be will actually bow the unsupported triggerplate upwards and jam the triggers or the safety or both. Thats why sometimes you find them backed out a bit to allow the gun to operate OK, but it will lead to stock battering problems in short order. Wether it will line up properly when tightened properly is another story. Get it tight first.
