This is an interesting situation! So the piece you see is not in your manual? I see a number of possibilities. I do not have a Westerbeke nor have I any experience with one. My answers and questions are based on deductive logic.
Here is my guess: Is there a non-working oil pressure gauge on your instrument cluster? Perhaps the sensor is missing here, or perhaps the engine features a port for it but it isn't used on your instrument cluster and the cap has vibrated off. If this is so you can install and wire the sensor or just cap it off if you can find a screw-on cap that fits the threads. Regardless, it does not explain why the feature is not in your manual.
And now on to the alternatives:
1) Someone has modified your engine. It doesn't seem likely since you are talking about an opening into the block from which oil sometimes escapes. Who would drill a hole like that? But from the pragmatic point of view, can you just cap it off? Perhaps there used to be a cap and it vibrated off.
2) You have the wrong manual. This, too, seems unlikely. Although the Westerbeke web site for manuals
lists two W30 engine variations, they have only one set of manuals to cover both. You might want to download the full set of three anyway since they may have updated the manuals since yours was printed/downloaded. They are about half way down the page.
3) You have misidentified your engine. You should be able to send Westerbeke your engine serial number and they should be able to tell you the model and variation.
4) You have a rare engine variation (or one that was not originally designed for marine use). I noticed on the aforementioned Westerbeke web site that they list a 30B and a 30C. This implies that there is a 30A, and maybe a 30 (which may be rare or may not be marine versions). This is not as unlikely as you might think. Believe it or not, I have seen a Kubota engine installed on a sailboat that was the tractor variation. I know the guy who was rebuilding this boat. He deliberately chose to use the tractor version because it cost so much less than the marine version. My advice is again to send the engine serial number to Westerbeke. Their customer service should be able to tell you which set of manuals to download.
Hope this helps.