Vacuum hose question
Installing the different vacuum lines and electrical that run through the the firewall prior to installing engine. 1973 convertible small block 4 speed. My question concerns the large rubber grommet that the headlamp control vacuum lines run through. Mine has 3 holes, photos I have of disassembly shows 2 lines passing through this grommet and the AIM manual shows 2 hoses. I've seen new grommets on line for this application that have 3 holes. What is the third hole for? I'm guessing it should be closed off somehow if not used?