In the old days the barrels were used for storing food, mostly olives, in the barrels while they cure.  The curing is a fermentation process which causes a  build up of pressure. The barrel lids were designed with a raised circle in the middle roughly 2″ in diameter that actually has a screw cap. A key would be inserted into the cap and turned to release the pressure. Product is more and more commonly cured in tanks now so the need for the “bung hole” to release pressure has become obsolete.  Most barrel manufacturers have eliminated the cap do reduce plastic production.  As a result some lids will have the bung hole and some will be smooth. We do not recommend using the cap. If it is open debris could fall into the cap area and compromise the screen, allowing insects into the barrel and other unwanted debris. We do not guarantee which lid you will get.