Gandolfi,

I don't know if this has been mentioned elsewhere, but many (relatively) modern process lenses were equipped with square stops.

The reason has to do with the nature of lithographic film--below threshold exposure, the no density develops; above that exposure, it goes to maximum density. A halftone screen produces dots which have smoothly falling edge brightness; the radius at which the brightness crosses the critical level determines the radius of the resulting dot and ultimately the average ink density on the printed page. Now, in theory, the dots are so small that the original image is reproduced arbitrarily well. In practice, because of diffraction (if nothing else) sharp corners are rendered as radii, and the halftone process exaggerates the result. A non-circular stop allows the effective aperture to be larger at some angles, thereby suppressing diffraction, while keeping the "average" aperture small enough to suppress lens aberrations.

The Penlite aperture looks to be a refinement of the square aperture, to allow vertical/horizontal and diagonal enhancement at the same time, probably to improve the "look" of certain typefaces. The need for this may be an indirect indicator of the overall quality of the optics---so you might just have a decent portrait lens there anyway

Incidentally, in the dim, dark, medieval ages of the electronics industry, masters for printed circuit boards and even integrated circuits were hand-cut from red plastic film, and to get uniform-width lines the corners were "filleted" on the inside and outside (a little notch at the inside corner and a corresponding point at the outside corner). This did more or less what your odd aperture would do; maybe the lens dates from that era (late 1950s through early 1960s).