What looks like a box out is actually the HVAC plumbing. I can see the floor joists in two locations, in my mechanical room, and my storage room, and I can see from one end of the basement to the other. The joists are 2x10 on 16" centers, house is ~28 ft wide. The home was built in 1956. The downstairs has rooms on one end with a center hallway, which appears to be the only thing supporting the 2x10's in the center. On the other end, the staircase comes down and has walls on either side of it that provide support (I suppose).
This pic is through wall section "A"'s opening, down the hallway. The hallway is matched upstairs(for the most part) but the upstairs hall is not as wide.
This pic is from the storage room, which is the first door on the left in the above picture. It is facing over the hallway through the joists, towards the room that is the first on the right in the above pic.
This pic is the staircase on the opposite end of the house. It is not centered, the hallway is. Either side of the stair case is a stud-frame wall, the right side is sheetrock on the stair side, and open on the other side (mechanical room). The left side is sheetrock on the stair side, and wood paneling on the other side (yuck, I know....).
Is it unsettling that there is no center support beam... or does it seem that the downstairs walls must have been put in originally to serve as support? Would it then mean that the Z shaped wall must also be a support?