And here is the back side of that two-bolt distributor, on which you can see the tiny, corresponding vacuum port with it's sealing O-ring in place. As the distributor bolts are snugged-up, that O-ring which protrudes above the flat surface is compressed, sealing the press-fit connection. DD
And here is picture of the adapter require when converting from a 3 bolt helmet distributor to a two bolt crab distributor (engine has a 3 bolt timing cover).... As seen on VanPelts site....