My pool was built in 1954. I have had it the last 20 years. I was told this was a plug that was never to open. Well I have a leak in my pool & the leak Detection people came to look & they asked what this plug was. So I told him- whenever he put helium down the skimmer bubbles would come out this plug, He thinks it is an equalizer of the skimmer. So 1st I need to know is it a plug or an equalizer for the skimmer? 2nd - how do I open it?  I don't have any idea about the skimmer (who it is made by) all I know is it has a JACUZZI collar on it. Help , please if you know anything about this. The guy can't pressurize the suction line until he can open this.  Meanwhile my pool is losing over an inch a day of water.  Thank you in advance ps. nobody in 20 yrs has ever asked me about this plug or even mentioned it.what is it