This is the last version prior to the Summilux Asph. Designed by Peter Karbe, it sort of transitions between ealier Mandler designs and the current Asph, which is also designed by Mr. Karbe.
It is a 7 element, 5 grouping design with a 12 blade aperture mechanism. This mechanism yields almost perfectly circular bokeh throughout the aperture range. You can expect half click stops between the aperture settings.