Yes Luca, The stem can be a one piece--with a flat button on he top or it can be a stem with threads on the top--the button screws on. All stems for these cases are different shapes-made to fit the hole and able to be depressed to open the cover.
Toward the bottom of the stem a small hole is drilled through the stem. This hole lines up where the bow fits into the pendant. The stem is held in place with a piece of wire fitted through the hole. To insert the stem, remove the bow with bow plyers. Insert the stem, place the wire through the stem. There should be a little bit of wire extending beyond each side of the stem, enough to catch and hold the stem into the pendant. Replace the bow. The bow will hold everything in place.
Stems are out there, but they must be fit individually to each case.
Tom