Good question. It’s my understanding that the spin (or ‘swirlies’) are a result of optical vignetting on out-of-focus highlight areas near the edges of the image circle. Essentially, the circles-of-confusion which are perfectly round at the center of the image circle gradually become more like cat-eye shaped ‘ellipses-of-confusion’ toward the edge because the exit pupil is partially obstructed by the lens barrel itself. It happens on nearly all lenses at full-aperture near the limit of their covering power, more so at wider angles, but especially in lenses of antiquated formulas.

Blade curvature has an effect on the roundness of the blurred areas, but in more of a round vs. octagonal (or round vs. pentagonal, etc.) sort of way.

The lens used in the 8×10 portrait is a remounted Wollensack from near the turn of the century.