The facts are these:
1. The spot is always centered horizontally and vertically.
2. The spot is basically a nice disc, but it can be slightly incomplete or irregular.
3. The spot is white, but it can be transparent to one degree or another.
4. The spot never appears in color photos and rarely in photos later than 1950 or so.
The evidence seems to show that the spot is an optical phenomenon and not (for example) something in front of the lens or a chemical accident at the darkroom stage. But at some point something in camera design changed to eliminate the spot forever. What was it?