Seems to be a variation...some with an o-ring for thr gauge to sit on, and some just selaed with "goop". Mine has no o-ring there and was never cut for one.
Found I never took a picture of the gauge block.
This is NOT THE SAME PART...has nothing to do with a Discovery....but it will give you and idea of what the gauge block looks like (except the Disco's is silver) and how it works.
Gauge block is a tube inside the air tube, o-ringed at each end, and bridges the hole in the tube where the gauge passes. Gauge screws into the inner tube. So if either o-ring on the gauge block leaks, will leak out of the hole in the tube, and will SEEM like the gauge's threads are leaking.