With the plug you close the entrance of the oil pump. Oil cannot get out and contamincation cannot get in.
Fischbein 11453 PLUG, OIL PUMP (11450)
Easy to loose, the red plug from the oil pump. It keeps the oil in and contamination (dust etc.) out. Bag Sewing Europe has them. This plug is for the fischbein Model F Bag Sewing Machine.