It sounds like you're missing the nipple.
Otherwise, where is this threaded opening, exactly? "Under the hammer" is a big place. Some have a cleanout hole, which is supposed to have an Allen screw in it. The cleanout hole is an extension of the flash channel on some caplocks, and goes at a right angle to the nipple.
In any case, an illustrated operator's manual is in order. You should call the manufacturer, who I am sure would be glad to supply one.