‘67 Barracuda vacuum gauge connection

Its supposed to be 1/8 I.D. tubing. No clamps requires tight fit to nipples. Thus the .16 O.D. of smooth nipple on gauge. Nipple at manifold has the small budge so it doesn't seize the entire length of nipple and will pull off with minimal effort, until it rots and breaks anyway. I would imagine their grommet was placed on the hose before placed into the firewall.