It is a change in the way the iris leaves are hinged.
Modern irises originated with the Compur EVS shutter in the
late 1950's. In order for the exposure to remain constant
the distance the stop and speed controls move must be the
same for an equivalent change in exposure. Its fairly easy
to make a shutter so that the speeds are evenly spaced on
the speed scale, but to match it with stops the iris must
follow an inverse square law since the stop is a square law
function of the diameter of the stop. Linear stop irises
usually have fewer leaves than the older variety. The old
type iris is composed of a series of crescent shaped
sections which are hinged on a fixed pin on one end and have
a pin running in a slot on the other. Sometimes the relative
positions of pins and slots are reversed but the idea is the
same, A rotary motion of a ring connected to the moving end
of the crescent shaped segments moves them together or
apart. The uniform spacing type iris has blades simiar to a
shutter. These have two pins at one end and are brought
together by tilting them. Its hard to describe but a diagram
makes it plain.
  The low number of blades on newer type irises results in
stops which are not very good circles. Some think that this
results in an unpleasant effect on the out of focus part of
the image, what the Japanese call bokeh. Ghost images of
bright objects take on the shape of the stop. Typically they
are pentagonal on newer lenses. Some old lenses,
particularly process lenses, like the Apochromatic Artar,
may have as many as twenty leaves or blades making up the
iris diaphragm. Most modern lenses have only five.