Do you push the plug into the the blank hole? Does the rubber expand in the hole?

Please see the diagram below, number 1 is the housing, number 4 is the plug but this is a metal threaded plug. https://lsengineers.zendesk.com/attachments/token/ANaRINc9pHIegbTmtd9IuwrDu/?name=4230+640+0506.png