Polaris SP350 350 Mcm Set Screw Plug, Sold By The Piece
https://nsiindustries.com/product/replacement-set-screw-plug-for-connector-350-mcm/
Polaris Multi-Tap Connectors feature removable plugs to cover the set screws used for tightening. This replacement plug fits into those ports to prevent debris, moisture and other contaminants from entering the connector.