How to plug the big block oil holes?

[davepl]

After a couple of years, I'd like to start my engine today, or at least soon. First, however, I need to re-prime it, which means oil, which leads me to:

Above the filter there are three holes. Unlike most pictures I can find, on mine the larger is toward the rear and the smallest toward the front.

The center is the factory oil pressure sender, I know that much.

One of them must be the clutch pivot ball. I'm guessing the larger, and it seems to go nowhere (I stuck an awl in it, not much depth).

So my question questions are:

1) What's the front little hole? I've tried screwing an oil pressure sending unit in there, which looks to be the same size, but no go. Won't start threading.

2) Did they reverse these two holes somewhere between Mark IV and Mark VI engines?

Thanks!
Dave