[salguod]

I believe that you are right.

I've attached 2 images, one of he block (with the offending broken bolt) and one of the housing.

I believe the oil comes from the pump out of the upper of the two small rectangular openings in the block. That aligns with a passage that leads to the outside of the filter. Oil flows through the filter and out the center, through the "bush" in the round opening. The oil pressure sensor is connected to the center passage, just before the bush.

There are spring loaded valves at the base of the housing where the passage from the pump comes up and in the bush that would only allow flow in that direction. There's also a spring loaded bypass valve in the bottom of the filter housing straight to the pressure sensor that would allow oil flow if the filter is clogged.

So, there should be little to no pressure or oil in the C shaped, larger part of where the housing meets the block. As you can see, there is very little oil indicated at the bottom of it (where my broken bolt is). The only access to this area is from the center of the filter, but the o-rings in the stem of the cap normally seal this off. I suspect the oil that has made it in there is from drips during 20 years and 240K of oil changes.

My plan is to put it back together with 5 bolts and hope for the best. It really can't leak much worse than the 300 miles per quart it was getting past the rock hard o-rings in the bush.