In similar locations on other machines I have worked on where there is a threaded connection that is not being used, there will be a solid brass washer under a threaded cap, with the brass washer having a gentle convex curve on the face against the threaded fitting. The concave face approximately replicates the face of a pipe olive, to ensure a seal against the fitting. Usually the cap will be solid, but the factory may have just used a nut that is usually used at the end of a pipe with an olive fitting - in that situation the hole in the cap wouldn't matter since the washer does the sealing, not the cap. The other possibility of course is that that fitting is meant to be an air release valve (since I can't see one anywhere else, unless that's one hiding behind the mystery fitting in the picture) and it is either worn out or has been reassembled with a part missing, although it doesn't look like any air release I am familiar with.
With the gauge, sometimes when the needles sit like that at rest it is just that they have moved on their shafts - sometimes carefully pulling them off with fine pliers and gently pushing them back on in the zero position will fix the problem. Although with that type of gauge the biggest problem will be removing the front face of the gauge to access the needles. If you do need to replace the gauge, a San Marino gauge is likely to fit (looks like same pipe connections and mountings, and they also fit in a rectangular panel cutout) and may be easier to source than a genuine Unic gauge.