Looks like this may be the issue, but the drawing in the link is very different from my block.
Looking down the sender tube there is clearly the outline of 2 mating surfaces about 8 inches down. This puts that point at about the centerline of the crank main bearing cap, but it's not a dead end, as the picture suggests. The port to the pump? Can't tell much more about what is going on by looking down.
I then pushed a wire into the 2 ports in the filter mount area, and both stop after only about 2 inches, and sure enough they both are visible in the sender tube. Cleary there is no plug, and this would certainly explain the problem I've got.
Now, how do I get it resolved?? As I noted already, taking it apart is not a simple task. The pan is bolted through from inside the timing casting, and that damn heat and press flange totally blocks access to the bolts.
I'm thinking - put it in from the top. It's a clear straight shot down, not much different from pushing it up, just further. The real challenge is - how do I know exactly where to put it??