Comparison of the positive and "negative" suggest to me that this is a Sabbatier effect (incorrectly called solarization) -- still-active developer after peeling, combined with bright light, can cause partial reversal. You'll notice that the very bright strip along the lower edge of the low building is dark gray in the negative, rather than bright white as in the positive; this would be a region that got enough initial exposure/development to be either insensitive or shielded by existing silver at the time of the reexposure. Delay in clearing (holding the negatives until the end of session to clear) could have contributed.
FWIW, this kind of effect is very commonly seen on the paper negative of non-P/N Polaroid B&W materials. Normally, the combination of the clear film base and clearing bath prevent it from occurring with Type 55.