Cultural transmission belts are the immediate sources from which the bulk of people adopt and use mental methods, ideas and value standards to determine how they think and act, ranging from daily life to consideration of more than that as well (contrast today's range of the moment attitude to the better thinkers among ordinary people in 18th century USA).
The operation of the belts is predominantly as perpetuators of the existing methods, ideas and values of the culture as taught to them by the operation of the same belt mechanisms in the past (ie the list provided by Andrew). This is but an elarged version of parents teaching their children what the grandparents had previously taught those parents (and they in turn by great grandparents, etc), all without adding to or subtracting from the content so on-taught. The actions of teachers, journalists, clerics, politicians, military officers training new enlisted men and cadet officers, business managers inducting new employees, and likewise all other persons with the responsibility to impart cultural institutions to their respective charges, enlarge this process further, to the extent that the adult audiences are still open to educaton and other influences.
The belts are also the distributors of new MIV's originated by thinkers of various calibres, ranging all the way from high-level thinkers (philosophers, mostly, whether full-timers such as Plato and Aristotle working expressly as philosophers or part-timers such as Aurelius using their philosophy as an element in their main work in other high-level areas such as politics or law or economics or art), through popular authors (Heinlein), journalists (Kronkite), editorialists (Beck) and frequently-reported-on advisor-types (eg scientists like Ehrlich, former politicians like Gore, etc) down to innumerable small contributors who toss in small ideas and reinterpretations of their own creation (eg small-time clerics, teachers, journalists and other commentators et al adding in their own 2c, with varying degrees of influence and varying degrees of dependence on the higher-level ideas from others for the bases on which their 2c additions are developed). All the ideas in the perpetuation-mode above had to begin in this kind of origination at some point.
The originators also work predominantly in a professional capacity in the belt mechanisms themselves, as academics or journalists etc, blurring the distinction between who is an originator and who is a transmitter because the individuals in question are almost always both, varying only in their dependence on and influence upon others.
This describes the professions and institutions in which philosophical principles (consciously held or not) intersect with everyday life, such as: