Stopbuchse

The "stopbuchse" is the graphite packing for the regulator valve spindle. If the stuffing box is not leaking, I would not loose any sleep over not having this part. If it does leak (indicated by a flame at the stuffing box when the stove is operating), and tightening does not resolve the problem, you might try some graphite valve packing material available at many hardware stores. Finding the original part will likely be a challenge.