It was also scaled to the focal length I was after. "The original design was tweaked to enhance the swirl effect. The only real definitive answer I can find to this is from Mats Wernersson's website who made his own swirly bokeh petzval.
I know obviously petzval lenses are famous for this but also a lot of projector lenses. In case anyone else is googling this, i know this question is old. I'm sure Marianne can give a more complete explanation. The inside arc of the 'football' shape is defined by the aperture, and the outside arc is defined by the edge of the lens element. In it's simplest form, an eclipsed exit pupil is just caused by the rear elements being too small, so that rays which make it through the aperture are stopped before they reach the image plane.
So what causes the exit pupil to be eclipsed? Retro focused designs? I really only notice the football shaped highlight bokeh on my 50 1.4, but it really doesn't swirl like the petzval lens. If a flat-field lens is used, you need to ensure the background is at a different distance than your subject, and the bokeh effect will be less emphatic, but it will be there. Strong field curvature helps by placing the periphery further out of focus. The Petzval lens is constructed to take good advantage of this. It is caused by eclipsing of the exit pupil (which, by the way, is also a primary contributor to lens vignetting).
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