I'd bet you are looking at the provision to mount a water temp sensor in the head. I don't know why it would have a vacuum fitting on the plug, sounds like someone put that in because it fit. I would just plug the hole and forget about it.
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65 Catalina sedan. Allen Thomas Performance 495. KRE Heads at 310cfm ported by SD Performance, ProSystems Dominator carb on ported Victor intake, P-Dude custom grind hydraulic roller, MSD ignition, 3.50 Moser/Ford rear. F-Glass front bumper by son Rob, rear by the old man and joint effort for trunk lid. 3950# w/driver. Best of 9.5761/139 on 175 shot, 6.01 /114 in 1/8.
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