This is a diaphragm dial that is a little different from the Tiffen dial. Like the Tiffen dial, it slips over the pins on the Argus diaphragm dial and provides a side reading scale for the aperture. It needs to be held in place by a Mil-o 19 mm screw-in to Series V filter adapter or else it will simply fall off. The filter adapter holds a Series V filter using either an insert ring or a lens hood. The adapter should not be screwed all the way tight against the diaphragm dial, or else it will pinch the dial against the face of the lens and the dial won't turn. Like the Tiffen dial, you need to put an aperture index mark on the lens.The advantage is you can read the aperture with a Series V filter or lens hood attached. The disadvantage is that the aperture scale moves around as the lens is focused. I got this dial in a box with a Series V adapter and a lens hood. The finish is bright chrome with engraved markings.
Mil-o was a trade name of Mr. Miller Outcalt (1912-2004), Hollywood, California, who sold photographic accessories. At one time Mr. Outcalt distributed the Asahiflex single lens reflex camera made by Asahi Optical in the days before the Pentax, and the Yashica twin lens reflex camera. Another of his trade names was "Kalt", which appears to be still in use for some photographic supplies.
